We have two fish ponds in our yard. We built the dragon pond before we put in the bamboo. Shortly after that it got clogged with bamboo leaves. It has been a problem since then until this year when I built a shade cloth structure to keep the leaves out. I was worried that it would be too dark, but the ribbon-weed survived, and the fish thrived.
The other pond was larger and had a plastic liner which lasted twenty years before it started to leak. Instead of concreting it as I always intended we decided to use an old bathtub.
Unfortunately it was smaller than the old pond and we didn’t have the soil to backfill around it so I filled the hole with leaves and hoped it would compost down in time.
This pond was working well for a while but then the number of fish seemed to decline. I tried feeding them more in case they were so hungry they were cannibalizing their young. This didn’t work so I tried feeding less as excessive feeding can poison the water. I even tried thinning out some of the rampant vegetation.
The puzzling thing about it was that the largest fish were disappearing and leaving the babies. I should have guessed that it was a predator. Then one morning I noticed two tiny snakes (no more than one foot long) floating on the duckweed. I think I had actually seen them before bot not recognized what they were and assumed they were part of the vegetation. When I disturbed them they went down a burrow in the half-composted leaves. It should be warm down there, and with food laid on they should grow quickly.
They are keelbacks, specialist freshwater snakes that live on fish and frogs. They are also one of the only native animals that can eat cane toads. I am thrilled to have such valuable creatures in my garden, and don’t begrudge them a few guppies. There are plenty in the other pond I can move across if they run out.