I did the same thing but it was many years ago when I kept several types of african fish. I used silicone seal that is made for use in aquariums. No pics but here was the process I used. First I removed the plastic frame around the top. Cleaned off the old silicone seal from the top edge and frame. Next I got a plastic extrusion, similar to the plastic on the back edge of the put together folders. The ones that have clear sheet front and back and after putting your printed material in you slide the plastic over the back edge to make a booklet. This type of material is available in office supply stores. I glued two pieces to the tank, one in front and one in back using the silicone seal. I then had glass cut to size. Each partition would slide into the plastic strips like a guide. The last step was to reattach the top frame with silicone seal. The fish stores already sell a similar product for keeping the colorful fighting fish, bettas if memory serves me well. I already had extra tanks and the silicone seal so I made my own.
John Q