A perpetual motion aquarium huh?
Even if you were only interested in getting some of the energy back, it is going to be a very small amount. Perhaps enough for a very small LED
. I don't think the energy or cost saved or environment saved is going to worth the monetary cost or effort and time required. Any environmental benefits will be off set by environmental costs of the manufacturing of materials used..
You can't use a photo-voltaic cell to make energy for the light that is the source of the light. You can't make energy from moving water to move water that is the source of the energy. Same for heat from heat. You're just not going to get enough energy from the limited amount of moving water to power any electrical device.
Your best efforts would be directed to making best use of primary energy source; insulating tanks, insulating and temperature controlling entire room; using efficient pumps; larger compressor, storage tank and distribution valves for air; using larger centralized water filtration and distribution where possible; one long light bar for several tanks; etc. One larger heater, pump, compressor, or light uses less energy, produces more effect than several smaller ones.( Two 50w bulbs use more electricity produce less light than one 100w) For maximum environmental effects obtain main energy from "greener " source.
But "just for fun" you could power some very small novelty thing from small a hydro-electric generator.
"In this house we obey the laws of thermodynamics!" Come on over to my house. On weekends sometimes we defy the laws of gravity. No success in actually breaking them yet, but lots of defiance.