I've heard about never-ending problems with micro bubbles with these things, but that's simply a matter of aesthetics, not damage to the tank or inhabitants. I've also heard about problems trying to fiddle with them a lot...but nearly all users seem to be putting them into the backs of their Nano Cubes and have a problem keeping the water level constant in the back area. I'm not sure if there would be as many problems if you put in the reservoir of an Aquaclear 500 or something where the level will always be the same.