But just for installing a drain box on the back of a tank for a bean animal overflow, you would use an adhesive rather than a sealant? I guess I might as well use RTV then.
I use Momentive RTV 103 black (RTV 108 is clear). You won't find it at Lowe's or Home Depot. It can be obtained through Grainger's, if you have a local outlet. Or, Amazon has it.