You can use a glass mixing bowl insted and a plait wate for the angle to swim in and place the plait over the entrance. It has to be a clear mixing bowl one they can see through or they may figure it out onthe other hand if they can not see through it and you can leave it in the tank for a day. On the other hand they may think its a new cave if they can not see though it and be at home with it then you can sneek up an them and cover it.

Bill