Well, it's an interesting idea and I admit to thinking about stuff like this myself once in a while. But the problem is I'm a huge stick in the mud on most ideas.
Here's what I don't like about the idea.
A hydroelectric dam is a hugely prosperous idea because Mother Nature (aka "Magic"
) puts that enormous amount of potential energy up high for us, and we can then parasitically reap the benefit of the falling water and make electrons move around.
In the case of the aquarium, you have a pump lifting that water before it has a chance to fall back down. So you're providing the energy for the energy generation anyhow, and it seems to me it's probably less efficient than just plugging in a battery charger into the wall. Those turbines are going to put backpressure on the overflow piping as well which causes the water level to be higher in the main display and cause a flooding risk if anything gets in there (small fish, snails, etc.)
It's sort of like putting solar panels under your halides. Sure, it'll generate you some electricity back but you're casting a shadow into the tank. If you could put panels above the lights and catch the light that's lost upwards then you could probably recapture some energy but it's going to be a tiny fraction of anything that again I question that it's isn't just more effective to just plug the charger into the wall, or a solar panel outside would serve you so much better because then you're capturing energy that's there for free without you spending $$$ to power the pumps or lights in the first place.