What a pest these little buggers are. If the leaves on a water lily are turning yellow, take a good look and you will see tiny black Aphids.
These buggers really suck the life out of plants. A good way to eradicate them naturally is to dunk the leaves under water, or use a high pressured hose to wash them off into the pond. The fish will love them. If the infestation is severe, a little trick is to wipe the leaves with a very small amount of olive oil and hose them off and then scoop them out with a net. Be sure not to use any insecticides as these chemicals will kill your fish.