My home came with an Aquascape style pond of about 1400 gallons. In order to get an accurate take on the exact volume, I had to do the salt test with the salt meter. When you have a lot of rock and boulders, it can be difficult to accurately estimate volume.

After two years of trying to cope with the rocks, I gave up. I would have the pond cleaned, or drain and clean it myself. And two weeks later, the bottom would be filled with black muck again. And I had an entrenched problem with milfoil weed. It likes to attach itself to the debris and bury it roots in the rocks and gravel. It was tedious and required help at the end, but I eventually got all of the rock and gravel off the bottom. Yes, a bare liner is not as attractive but the water clarity improved overnight. The milfoil disappeared. And the water stayed clear until I got invaded by green algae.

My original set up has a skimmer with a mat, net and brushes, and a 4900 gallon per hour submersible pump. It has a waterfall box that has mats in it. I replaced all the original mats with Matala mats which are much easier to handle. And as long as I didn’t have green algae, the water clarity was pretty good. But the last two years even a submersible UV filter was not enough to keep the green water at bay. I did work at keeping the fish load light, eventually settling on six medium to large koi in 1800 gallons. Some weeks were better than others, but it still wasn’t under control.

I installed a retro bottom drain with an aerator, but was lucky because there was a primitive version of one underneath all the gravel. So there was a pipe that ran through the side of the pond into the front of the skimmer that I could work with. It seem to help keep the water clearer after I installed it, but I still wasn’t happy with the day-to-day clarity year-round.

So this April, I installed an OASE 3rd gen Filtoclear 8200. That’s the maximum size, and it’s good for about 2000 gallons if you have a regular load of Koi in it. It might filter a larger pond if there are only goldfish or no koi. But I learned from their chart that when you actually have Koi, you need to have a much larger model and UV light than you might initially think. Their literature was the first time I ran across this concept.

After the gardeners dug a hole and I dropped one of their submersible pumps in the middle of the pond, it was pretty easy to hook up. Buried it just outside the pond and covered it with a faux rock. Pretty much an invisible set up. And since then, my water has been clear every single day. The UV light, which is 60 watts, did the trick. No green water at all this year. It’s extremely easy to flush the water through the system and takes less than five minutes a week. It also forces quite a bit of water out during that flush process, so I just add water back and my water change of 10% is done at the same time. It seems to pull quite a bit of muck from the pond so it’s also easier to rinse the net, brushes, and mat in the skimmer.

I echo the advice you’ve gotten above. Water garden style ponds with rocks on the bottom are impossible to maintain with koi. They collect muck, anaerobic bacteria, plants and debris, and are impossible to keep clean with a normal koi load. Adding an extra filtration circuit after getting rid of the rocks is the next best step forward.