That's the thing about bio filters...testing the water is the only way to know. So you're on the right track.

As the koi grows things may change. Just keep an eye on ammonia and KH and you'll know most of what's going on. In the first month of so testing every other day is a good idea. Maybe even nitrite too. You may never see any ammonia or nitrite depending on what algae do. If you never measure any ammonia it means algae is probably consuming the ammonia but some probably gets to some bacteria too.

After say the first month you could scale testing back to maybe once every week or two because it is such a small pond.

Enjoy.

Thank you so much. I am in the process of expanding my pond after doing research I learned that its untrue that Koi only grow to the size of their pond. The Koi I have now are babies only 3 months old. By the time I am done I am hoping to have about 800 gallons I know its still a bit small but its much better than what I have. If I have the lava rocks on the bottom of the pond will that be enough of a biofilter? Or do I need something else like a skippy biofilter?

Lava rocks, or any rock, at the bottom of the pond shouldn't be considered a bio filter. It is more likely to add more ammonia than it would remove over time because waste collects, fresh decay produces ammonia. The bacteria that convert ammonia can't work when covered with even the smallest amount of settled muck. For a time the bacteria can survive on the under sides of the rock but the problem is there's no real water movement so the water there will be very low O2 and the ammonia converters need O2 as well as access to ammonia.

I'm not an anti-rock person, or even anti-muck. People should have a pond they like. But I don't think we should kid ourselves either. I like the look of rock but don't like the muck downside so I mortar rock over the liner. That makes cleaning the pond easy. But even then I wouldn't consider it a bio filter. Yes, it is home to some ammonia converters, but not enough for me to consider it a bio filter.

Skippy is a really old design. It was king in it's day. But much better filters have been developed since then. Much better performers and easier to build. These include Trickle Tower, Moving Bed (fluidized bed) and Bakki Shower. And for mechanical there are sieves, sand & gravel and fabric filters. Bio is easy, mechanical really is rather complex and expensive. For Water Gardens I think a vacuum system is more appropriate in many cases. I think many people really kid themselves into believing some simple filter works for mechanical.

Like the Skippy. It is still pushed by many first time type people who's pond really didn't need any filter. They're in a sweet spot of being complex enough that people feel proud to build them but simple enough that anyone can do build one so DIY people love them. But if you look at people more focused on fish you won't ever see a Skippy. The issue is exactly the same as rocks on the bottom. The Skippy is suppose to be mechanical AND bio, but that's an impossible. Collecting waste and supporting ammonia converters in the same media is not really possible. But that depends on each person's definition. If a filter collects say 1% of a pond's waste some people say that's a great mechanical filter. I don't. The Trickle Tower, Moving Bed and Bakki Shower filters were tested against Skippy type filters and converted about 30 times as much ammonia for the same size. So to me Skippy, or any submerged static media filter, isn't a serious bio filter.

And that all goes back to whether koi grow to the size of their pond. You're right, it is a myth. But like most myths there is an element of truth. Virtually all small ponds with koi share common features of very poor water and little food. And that slows a koi's growth a lot. This is also true for almost all Water Gardens with koi no matter the pond size. It takes a lot of food for koi to grow to their potential, like being fed 4-12 times a day. That generates a lot of waste that must be handled. Growing great koi in a small pond is certainly possible but takes some serious know how and a fair amount of work. But most people with Water Gardens don't really want to grow their koi to be over 30" either. 12-24" is fine for most people.

I'm new and don't even have a pond yet, but won't the lava rock be a problem for the fish? I'm trying to learn for myself. I think I read somewhere (I've been reading a lot about ponds lately), that the fish could cut themselves on it.

Yes, some people do worry about fish injury. Mostly koi keepers. When a fish cost $100, $500, $5000 people worry about such things. In a Water Garden with some goldfish I wouldn't consider the risk very great. They don't have the mass needed to do a lot of damage to themselves. But I also don't see any benefit to adding lava rock either so I personally wouldn't.