So far it seems fine, fish are normal and water is testing ok. It is hollow, but mine had a small hole so it was able to fill with water. The only problem I have seen with it is that it needs something sitting behind it to keep it pressed against the glass or it will move around a little when putting the front magnetic piece back on. It could be a risk of crushing the fish if it decides to swim out if its not anchored well when you put the front back on.