I just managed to stack using the holes and creases. I wasn't able to get glue but it's not moving lol and the goby swims through holes I didn't realize I made.
I would get some putty to secure that one. If it falls it looks like it will break some glass. Did you switch the tomato clown out for something suited for a 10?
You could probably do a clown goby, hi fin and a couple of the neon gobies but you'd have to get rid of the dotty as well as the clown (probably a good idea any way as I wouldn't do a dotty in anything smaller than a 20 long)