I'm sure it's not for a lack of trying either, but I can only imagine that a lake with fish in it for thousands of years has something that a concrete and glass/acrylic box with some plastic decorations and gravel doesn't. Main thing I see is plants. Probably would help the water quality a LOT to have a refugium with native plants in it just to circulate some water through. Maybe 500g an hour in a 1000g refugium.
This is one of the largest american native tanks I've personally heard of, most of them that I see are 3-6 thousand gallons. At 20, with the proper filtration this one might have a better chance. I see tons of wide open space for the ones that I see at, lets say bass pro and gander mountain. Not only that but, it's not crammed either. No competitive aggression, and tons of wide open space. I would say it needs a few more added in for the bluegill species, some smaller ones too. Quicker and able to eat up all the smaller stuff leaving less to decompose and give water problems. Downside is making sure they get enough food to not be malnurished.
