It's a 5 gallon reptile tank I believe. The top frame is lower on one side, as if to accommodate a sliding lid. I was going to use this tank to grow some crypts emersed but I might want to use this tank full of water one day. The silicone is a bit thin in some places.[/QUOTE]
This should be easy to do. Get a razor blade scraper(the type for removing paint and/or stickers from glass. I like the ones used for removing inspection stickers from car winshields. Scrape all the old silicone from inside the tank. You must remove all of it-sides and bottom as new silicone will not adhere to old silicone. DO NOT DIG THE OLD SILICONE OUT OF THE SEAMS BETWEEN THE GLASS PANES! THIS IS THE PRiMARY SEAL AND IS WHAT HOLDS THE GLASS TOGETHER. Just scrape the old stuff away keeping the blade flat to the glass. When it is all removed wash the tank with white vinegar and water-dry the tank thoroughly. Get blue masking tape and mask off the joints to be siliconed-they don't need to be too wide. Use another tank for reference. Get some GE I silicone in a chaulking tube. The chaulking gun method gives the most control. ONLY USE GE I SILICONE FOR WINDOWS AND DOORS AS IT DOES NOT HAVE AND ALGAECIDES IN IT. BATHROOM CHAULK WILL KILL YOUR FISH. Run the chaulking along all the masked beads at once, it is do or die, do not let the silicone dry and then start again. After the beads are laid, smooth them with a finger dipped in either water or alcohol. Pull tape of immediately. let the tank dy for 24 hours then fill with water to check for leaks. It is easier than it sounds.