this really is common sense. don't make this that difficult. the other explanation is somewhat confusing but correct. just get a tape measure and from the short side of the tank measure the distance on the inside between the two long sides of tank. it is about 12" but don't use that. then subtract 3/16" from that so you have some clearance. it fits in better. also get the divider about 7-8" tall and glue it in at an angle so easier for frog to climb in/out. run a bead of silicone on both sides and be sure to test it for leaks before adding the substrate. let the water sit for a day or two because a pinhole leak takes awhile to notice.