If so, I strongly recommend the product listed below. However, don't take my word for it and, if you can, perform your own research on the best aquarium sealants.
ASI Aquarium Sealant, CLEAR. High-Purity Silicone Sealant, RTV curing. Safe for Fish and Continuous Submersion. UV Resistant. Non-sagging. 10.2 oz tube. Price/Tube.