Or UV filter.

If your stated pond size is correct your water volume is over three times the 1500 gallons. Can't help but saying I doubt the added rocks had anything to do with the resulting clear water. More likely the result of the pond supporting only a few small Koi verses the 35 assuming much larger Koi that died last winter. Only time will tell I suppose.

About half of the pond is only 10-18" deep, the center goes to 32". All I know is it's basicly all see through.

I am a far cry from expert, but so far my pond is pretty clear, which is a first. So I'm thinking, when the extreme heat gets here which so far hasn't gotten here yet, it'll change and the sickening green gunk will be back.

Wow I don't think I've ever read a post like this!!! Encouraged as I am now rocking my first pond and know about the anti rockers of the world! Stacking and Bonding my rocks to the verticals, Kewanee flats for the shelves and very little gravel or river rock. Only enough to entertain the fishes!

Hwarrick:
So you also have toads that also reproduce in your pond? I have a ? that I can't figure out: the toads mate in the pond, then I have 100s (1000s!!?) of polliwogs in the water that develop bigger by the week but still with a tail & no legs. All of a sudden one morning: every one is GONE... where do they go? Not developed enough to jump out, the goldfish leave them alone & no snake or other critter could eat ALL of them just overnight?!! Now another batch of teeny ones again in the pond after the visit by the toad pairs. Any idea how they disappear just overnight... EVERY ONE of them?! No snakes or birds doing it I am quite sure as it happens just over night one night!

I do know when they change it is amazingly fast. You have to be looking pretty close to notice the change while they're in the water. And when they decide to leave...they leave. With mud ponds I've had I have seen a bunch of them just out of the water and they just head out into the surrounding landscape. Seemed like it was just one day.

yes they do change fast, one day the little tadpoles are all over the pond then you have tiny toads all over the lawn maybe 3/8's of a inch big.my grandaughter loves them. and yes as the heat sets it does get harder to keep the pond water as clear as earlier this year. we just keep trying.

Oh, thanks for solving the mystery... it just seemed impossible they could mature & leave SO fast!! We now have ANOTHER batch of teen ones in the pond. Our city yard must be LOADED with toads by now! I can pick them up & the dogs seem to ignore them. Am afraid in spring when I dig up the garden I may strike one that had hibernated there !

I also thought that i had the green licked. I used the bio spere balls that you put in once a month they worked until abought 2 weeks ago now the green is back. I think it was clear becuse it has been cool this spring.

Yes now that the temps are higher the green is back, still working on the issue. can't help but wonder when the fish swim at top speed in the greener water, do they see enough to stop before hitting the wall. Also at feeding time I have one fish that guards the skimmer for floating food before it disappears into the skimmer. Smart, it's all soft by the time he gets it. Snakes, I can't stand them.