Well here are my thoughts.
1: Could you fix it, to add your own water back into the tank. I dont trust that It will remove all the toxins from the water.
2: How fast does it remove water from the tank. I know my fish like to swim real close to the siphon. Also, if it takes water out very quickly, you wont have much time to clean up the gravel.
3: How fast does it return water to the tank. If its to much pressure your going to end up destroying your aqua scape.
Overall, it looks quite handy. But I'd be leary of it.
-Jay