After many years of having a green pond, I had a large filtration system added of sponges and balls etc to my already existing Filter system and U V light. My pond is now clear but still dark particularly if the sun isn’t shining. If the sun is shining you can see the bottom. Any ideas how I can make it less dark please? The lining is fairly black from all the algae over all the years. Are there any fish that would help ?