Great news

What is Aquarium calk? and how much is it? I think there is a small space they can fit through between the divider and the top of it.

Also my 55 gallon tank came with a place you can insert a divider but I guess the pet store doesn't sell the dividers that would go in this space so the employee gave me one for a fish tank type thing and i dont think this is going to work. Unlesss the surface for it will be too smooth and the spiders may slide back down but I doubt that.

So what I may have to do is go to home depot and see if they can cut me a thin but thick enough piece of wood that stay in the divider hole that came with my tank. So I guess that means I''l have to measure it.

Ok so i went to home depot got what i thought was the correct measurements for the divider. got the wood home and the width is suppose to be 11 inches either I measured wrong or the guy cut the wood wrong because im short a few inches on the width so now i gotta put some kind of board or something there to make it fit tight enough so it cant be knocked over...this is frustating but its worth it so im not giving up.

I meant waterproof silicone caulk when I said aquarium calk. How does the lid attach to the tank? Is it hinged, or does it slide in and lock? If it is hinged or just lays accross the top, you might be able to put a strip of wood on top of the divider so that it sits tightly against the lid. That should keep one T from climbing over to the other side. You definitely need a flush fit on all 4 sides of the divider if you don't want one to go to the other side.

The kind of tank I got the lid just lays across the top of it however when I brought the tank i brought clips to hold the lid down tight so they get out..
so one of the boards i got from home depot should I put it between the lid and the top of the divider? That's what your telling me? if so I'll still have space left on one side of the divider because its not wide enough.

If the divider fits right up against the lid when it is down, don't worry about it. You just need to make sure there are no gaps anywhere around the divider, or it won't work.