Bad,
Glad to see another "boater" in the hobby. I've been working on large boats for over 20 yrs.
I use marine sealants religously around the fish and never had a problem. The only one I won't use is Boatlife's polysulfide.
I've used Sikaflex, 3M, West Marine Brand, etc, etc. Never had a problem.
Polyurathane's won't leach anything into the water. Normally just like on a though hull on a boat as soon as the sealant starts getting firm I'll add water.......... never a problem.
Be careful not to bump your through hull that you installed as nothing really sticks to Rubbermaid stock tanks. Marine sealants will create a seal, but not adhere very well. I have a cockpit drain that I use in one of my Rubbermaid tanks for a bottom drain, and every so often when I bang it from moving the tank around, I'll have to pull it, and re-seal it.
I've never had good luck with 3M products or any urathanes sticking to damp or wet surfaces. Boatlife's polysulfide is the only sealant that I use underwater, and it sticks better underwater than it does dry surfaces. The water makes it cure almost instantly, so you have to be fast. But beacuse it contains sulfides, I've never used it near fish.
I am the only one I know in Nor. Calif. that will install thru-hulls/transducers while the boat is in the water, I've done hundreds of them and never had one leak. My arm is the only thing that gets wet too.

But it is my trade secret, and I don't divulge how I do it

Most boat owners want to watch till a 3' tall 2" water column starts shooting up in their bilge, then they normally go for a walk
I am very "anal" about fixing leaks, I can't stand-it......... You and me both, being around boats teaches a person alot about keeping water out, as repairing the damage is never fun.
