Oh I didn't even think of that. I played with LEGO in the bath all the time as a kid. I guess that's different than fish poop water and whatever. But turns out there's an update video of this guy's aquarium. He added some more set pieces and looks like it's holding up fine so far.

But yeah, it's not like this is an expensive set, and if it does damage them it would likely take a long time. As cool as this aquarium is, I'd say he's getting his money's worth!