Lowes or home depot. 100% silicon without the mildew resistance in it. Next clean inside of tank really well. Tape all inside edges with masking tape leaving a 1/4in space. Caulking gun should have a picture of how to load caulking gun. Lay down a steady bead of silicon. With a finger or they sell the tools to spread the bead out neatly spread out the silicon. Let it dry just a lil while then remove the tape and let the silicon dry. That's wut I did to my 135 gallon