Green water is considered very good for goldfish. It is nearly impossible to get rid of from what I hear. There are ultra violet lights I hear can do wonders though. Best course of action to keep it low and get better visibility is do large water changes every couple of weeks. By large I mean get the water low enough till it reaches your fishes top fin. Goldfish can with stand large changes like this as they are not only pond animals but stream animals as well. When in natural ponds there are plenty natural springs, natural pond water never gets stale as artificial ponds. I do 90% water changes in my aquarium every week. Do you have a way to drain and fill your pond? There are water siphons that connect to your sink for easy water return so you can temperature match to your pond water. Just make sure to add appropriate water conditioners such as Prime.

A pond really should be getting round the clock filtration. Maybe you can hook it up to electricity during off hours?

The concern is also that even though the plants give some oxygen there is a give and take and right now the plants, algae, and fish are all competing for oxygen, then for 16 hours a day they are shut off of filtration which provides nearly all the oxygen in your pond. This could be causing lots of stress on your fish and can be the reason why your plants are dying and why your pond has lots of algae. Although even with the best filtration green algae can be prolific. Here is a link I found on this below.

Your fish are likely eating the heck out of those roots, I know my goldies loved em. Could not tell you if this was the cause of their demise as mine likely died as I had them inside the house.

Here is a link that might be useful: low oxygen levels in goldfish pond

I was wondering if you were using an appropriate sized filter as it wasn't apart of your pictures. For a 500g pond the preferred filter size should be 50 gallons. I am just learning about all this filter stuff myself as I'm building my first ever pond soon. Below is a link to a DIY filter project to elaborate what I mean.

Here is a link that might be useful: skippy filter assembly

The forum has been here before with regards to green water. You need a biofilter. It has to run continuously to keep the aerobic bacteria in the filter alive. This link is a good read.

Here is a link that might be useful: green water

The first step is finding out the source of the problem. Have you done any testing of the water? The biggest cause of algae proliferation is excess nutrients in the water. Eliminate that and you are halfway home. A test kit is relatively inexpensive.

Particles in the water can be from many sources other than algae. Scoop up a sample in a clear container and allow to sit for a couple of hours. Debris will settle or float out so you can identify it better. Filtration is absolutely a necessity.

Norm Meck's Green Water article is excellent. Thank you kalevi for posting it. Shade can reduce although not eliminate algae. Dead algae in the water makes nutrients for more algae. Filtration is a necessity.

UVlight and filter is a good investment for extreme algae proliferation.

A Skippy filter works very well.

Algae will attach itself to any surface. The roots of your plants are perfect.

Water changes just start the cycle over again. Correct the source of the problem.

The hornwort/anacharis is not "choking off" the algae because it's a myth that it would. Virtually everything you hear in forums will be myths because they're short and sound good if you don't think about them too much.

Norm Meck's article is just about the only thing out there that discusses algae with any logic and data. Virtually no one in any water gardening forum reads it. Much easier to just repeat the same myths over and over.

Sleepless says Norm's article is excellent and repeats the first myth, about excess nutrients, as fact.

Another myth is that water changes start the cycle. Many Koi pond keepers do huge water changes all the time without causing algae problems.

Skippy filters are only used in hobby water gardens. You won't see a single serious Koi keeper using them and no fish farmers. Water gardeners swear by Skippy only because they never test results. I have a Skippy, my pond is fine, therefore the Skippy must be the best filter. High end Koi keepers and fish farmers do tons of testing. And because of that data they moved to much better bio filters years ago. Search Trickle and shower filters. Easier and cheaper to build than a Skippy too.

Norm's theory is heterotrophic bacteria release a substance during aerobic decomposition that kills living algae. Trickle type filters are way better support for aerobic bacteria than a Skippy.

Norm's theory on bacteria is just a theory, but pretty good. And his experiments prove there is some substance in the water that kills single cell algae in a very short time. I have an add on theory, that string algae produce a chemical that kills single cell algae, and also that single cell algae produce chemicals to kill string algae. Volumes of studies over many years on this in the marine environment. I see no reason why it wouldn't be true in freshwater. And I've done experiments that lead me to believe this is true. Such as putting algae from a clear pond into water from a green pond and having that water clear in about one day. However, sometimes it was the string algae that died.

My guess it is both bacteria and algae are producing chemicals to kill each other. String algae and bacteria could be working together. It's been documented in marine algae. Just a question of who wins.

Best way to kill green water is a UV. Normally it only has to be run for a week or two. After that the bacteria and/or macro algae get the upper hand and a pond will stay clear for a very long time (years) with the UV off.

To help the process, a stream and/or Trickle type filter (there are several kinds). Often these are enough, no UV filter needed at all. But because this is a chemical war outcomes are not certain. UV filters are 100% certain.

This chemical war doesn't happen with nitrification bacteria since they don't complete with other animals or plants. Ammonia doesn't fight back.

No kind of filter removes string algae.

You can do water changes to remove small particles but a pond with plants is always going to throw off bits of matter. The only plant I keep inside the pond are lilies. Everything else goes into their own beds around the pond. Makes it easy to remove string algae by hand. Submerged plants like hornwort/anacharis are completely worthless. They have no benefit (they consume O2 at night), look ugly and just add debris to the pond.

Gravel bog filters are also pretty good at reducing suspended matter and can kill green algae too.

I know this was a complete waste of time, but every couple of years I like to chime in with my little rant. Now everyone can get back to repeating myths and leading ponders on wild goose chases.

Thank you for your input. I didn't realize I was repeating a myth as excess nutrients have usually been present on the rare occasion my pond has had algae.

I suggest a Skippy as a filter because it is uncomplicated, easily cleaned and relatively inexpensive. That doesn't mean that there are no other good filters on the market. Sadly, I don't have the money to install and test out multiple types of filters. People who raise Koi exclusively are often willing and able to invest the money in higher end equipment and that is certainly their prerogative. I wish I could do that but I am a housewife hobby ponder who has to watch my budget closely in order to have a small pond at all. I have to judge my pond by the health of my fish and the health and clarity of the water. While the claim that Skippys never need cleaned isn't true so far as I can tell, they do a good job of removing debris of all sorts from the water. So far as I can tell, all filtration systems work on the same principle, the differences are how they are set up, effectiveness, durability, availability of materials and cost. Complications are not always beneficial to everyone since their situations are so different.

I didn't say that water changes cause algae. What I meant was that without getting rid of the existing algae and correcting the original source problem, it simply continues to reproduce all over again. If the use of the word cycle was inappropriate I apologize if I was misleading anyone. A small amount of algae is normal and not a cause of concern.

I agree that a UV is the best way to kill free floating algae so long as it is sized correctly and used properly. String algae can be reduced manually or with Hydrogen Peroxide. The only time I have had excess algae was when I was experimenting with the small ponds or when it is carried in on new plants.

How come you haven't been posting? You used to be a regular. So far as I can remember your input was not usually ranting. The best discussions can get intense. While you didn't always agree with everyone, that is normal here and if I remember correctly you are a commercial pond builder who deals with certain types of equipment so you are familiar with the stuff most of us never get to use. For us, there are no two ponds alike just as no two ponders are alike. DIY is rampant. How about posting more and giving the rest of us the benefit of your experience? Discussions needs different input to be beneficial or it is just a lecture for a specialized group. This forum doesn't fit that label which is why it attracts those of us who are new to the hobby as well as those who are willing to share what they have learned. Don't get mad, just share your opinion. True, not everyone will agree with you but discussion can be very helpful to everyone.

Sleepless, I didn't really mean to single you out. What you said could have been said by virtually anyone in any pond forum. It is the standard line, has been for many years and will be for many more. Your post just happen to be the one I read before mine. I did think it interesting that you praised Norm's article but held an opposite opinion to his conclusions.

I'm always curious what "excess nutrients" means. I read it a lot but why don't people post levels of each nutrient? I always assumed it was because they didn't test. If algae is present the assumption seems to be there must be "excess nutrients". Presents of algae is the test?
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Clear pond goes green. Didn't the algae consume a lot of nutrients to grow? That is how plants use nutrients, to build cells. We're talking about a massive amount of plant material. So the nutrient level should be way less. Why don't the algae starve?

Green pond suddenly clears. The 2 Canna Lilies in the corner sucked up all the nutrients and starved all the algae? Laughable. When all those single cell algae died what happen to them? They decomposed, like in a couple of days. And what chemicals were released in their decomposition...the very nutrients that algae use to grow. The nutrient level will be higher after a pond goes clear. So why doesn't the water go back to green? Because it has nothing to do with nutrients.

Water gardens always have plenty of nutrients for an algae bloom. Higher levels only means super green instead of regular green. Clear requires a virtual absence of at least one compound. Impossible in any water garden. Very easy and inexpensive to test. But who's interested in that? Like no one.
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A Trickle Tower can be as simple as a pile of rocks with a hose pouring water over it. To me that's less complicated and less expensive than a Skippy. And they're way better at removing ammonia and higher O2 than any submerged media. Again, very simple to test with a bottle of ammonia and an ammonia test kit. In water gardening no one is interested in doing that simple test that would lay to rest hundreds of hours worth of posts pushing the Skippy.
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I haven't posted here much in the past 10 years or so because it became pretty pointless. I have little use for opinion, my own included. I prefer tests, data, that kind of stuff. People in Water Garden forums seem to like myths and not really that interested much beyond that. There are other forums I read. I don't post there much because I generally having nothing to add, but I learn a lot.
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BTW, string algae can be controlled with hydrogen peroxide...gallons and gallons of it. You're left with an even bigger mess of dead algae, but it will indeed kill it. In the lab it will also kill single algae. But again, gallons and gallons applied daily, for long periods.

Here is a link that might be useful: Hydrogen peroxide and string algae

Actually, in my experience with my original test ponds, string algae can be easily killed with a direct application of Hydrogen peroxide. It really doesn't take much unless you want to treat a huge pond with a huge amount of string algae. If it is submerged, you can get the H2O directly on it by pouring it through a section of PVC pipe and blowing through the pipe to force it through. If you do have huge amounts of string algae, you need to pay more attention. Even then, a toilet bowl brush does a good job of collecting it. Shade helps too.

Complete test kits are commonly recommended on this forum. They run about $25 to $30 dollars. I buy a new one every year and use it. Perhaps the presence of excess nutrients at the same time as algae is coincidence but since I rarely have either I can't claim to be a good test case. My biggest problem is PH levels which I control with Plaster of Paris slugs or baking soda, in an emergency. Is that another myth? I also use Activated Charcoal and Ammo-chips in the Skippy. I don't use Matala mats because Quilt batting is cheap. Does it work as well? I don't know but my water is crystal clear.

One thing I have learned on this forum is that no two ponds are alike. Another is that there is often a less expensive way to deal with a problem than the latest technology. Tech is sometimes just throwing money at a problem that could be avoided in the first place with a little thought and minimal effort. That doesn't mean that tech has no place. I am the first to admit I am a nerd but I understand there is a place for old tech as well as the new.

BTW, there is a Koi forum now on Garden Web that might interest you. I drop in occasionally although I haven't posted since I am not a purist. They might enjoy your input too.

My experiments with string algae also showed direct application of hydrogen peroxide did kill the part of the plant in contact with the hydrogen peroxide. Myths almost always have a grain of truth. Hydrogen peroxide does kill string algae. But that is not the same thing as being an effective treatment in a pond environment. Even in a small pond we're talking gallons of hydrogen peroxide and a lot of effort.

Test kit for nutrients? You said "excess nutrients" again. What is that? Phosphate levels of 10 mg/l? 5 mg/l? 1 mg/l? CO2 levels? Or just nitrates?

Plaster of Paris and baking soda do buffer pH. Easy to test as I'm sure you have. Same results over and over. So far I've never read anything that suggested baking soda didn't buffer pH.

I will check out the Koi forum here. I don't think I ever have. I'd have nothing to add as I know little about their hobby. But some of what they do can be adapted to water gardens and they're always on the leading edge.

Unless the dates are messed up the Koi forum is a ghost town. What is there seems to be about water gardens.

I've never heard of anyone on this forum needing to use gallons of Hydrogen Peroxide. Even on the rare occasion I have used it, it only needed less than a cup. Application is direct, not general. I still have a bottle I got 3 years ago. It only works for about 4 or 5 minutes before it is too diluted but that is enough. Why treat the whole pond for a very localized problem? And it doesn't hurt the fishies.

Geesh! Even some of the test kits I bought compared the existence of high nitrates to excess nutrients. Some of the causes can be too much free fertilizer, too much food, too much poop, and rotting debris. PH and buffering is another important subject related to water health and testing.

Sorry about the paucity of posts on the Koi forum. It is a specialized topic. More involved and knowledgeable people posting to share would be helpful to increase it's activity and interest in a geometric fashion. On the other hand, this forum has always included those same topics whether or not the posters are novices or experts. It may be less intimidating.

If you treat string algae when there is tiny amount, like the size of a hand, I guess a cap full might work. I saw a cap full kill a patch a bit less than the size of a hand once, but the rest of the plant kept growing.

The cases I see posted are more along the lines of a 1000 gal pond full of string algae, stream, waterfalls. A person could go around treating each tiny patch with a cap full but I think that would add up to gallons of H202 and hours of effort.

I don't think it's very effective because you'd have to kill every cell, or it would come back in short order.

I'll also say I only once did I see some string algae killed by H202. Several other times I saw no effect. Including pouring 1 pint on a patch of a turned off waterfalls. It is possible that different species are effected differently or effect is different at different points in it's life cycle.

Unfortunately I doubt we'll ever know since there seems to no interest in discussion on the subject in any pond forum. Only endless bickering trying to win one's point with words rather than data.

Well, if you include the FAQ, there are 233 threads that refer to string algae if you do a forum search.

Well, nice to see that a few folks are typing over here. I was thinking this forum was almost a ghost town too. It didn't look like the OP had ANY serious problem and in any case not a string alge problem.

But since this has devolved into string alge, personally, I like it. It helps to filter the water. Admittedly, my 4+ yr. 5X10' goldfish pond has the uncommon advantage of being connected to a 25' stream with lots of plants. I have occasionally seen gobs of string alge in the pond, but generally it is mainly in the stream catching all sorts of crud.

As I have said before, I occasionally grab a toilet brush and snag a few globs of it and sling it into the woods. This removes the alge and the entangled crud. This does disturb some crud and this clouds up the stream for a few minutes and the pond for a couple of hours until the stuff settles or is retangled in the stream. Otherwise, I always have a completely clear stream and pond. I have a small Skippy. I suspect that it is not doing anything as I had to clean it out once looking for a leak, and found that it was completely clean!!

Other than that, I have NOTHING else. No skimmer, no filters around the pump, no uv, no nothing, no problem.

chas045, I agree on all points. Streams are one of the best filters going.

I guess the ponds with string algae don't have koi???
We only had string algae the first year we had our pond,
when our koi were small.
The next year the koi took care of it...they eat it like
candy.

Donno how a skippy filter works either, but for koi you need proper filtration and that doesn't mean expensive.

We maintain a clear pond with a DIY BD gravity fed system,
and incorporated a biological pond the second season.
We don't have a uv light, never use any chemicals, except
for salt in the late fall.
There are many ways to maintain a clear and healthy pond,
and our way in only one way.

Thank you, my koi have also been great for eating string algae ...and hyacinth roots and taro and water lilies, etc.

Yes sleepless, it's not so hard. Keep the filters
clean, and do frequent water exchanges.

I get a real kick out of some koi keepers with their
super duper expensive systems, and they still have to rely
on uv lights and chemicals to keep clear water.