It's not sealant, Roc...
It's more polyethylene. Tank mfrs use a process called "spin-welding"...a kind of heat welding, which is the only thing that works. A spin welding tool-- which kind of a combination highspeed drill and hot glue gun--melts about half an inch of the edge of the fitting and about half an inch of the edge of hole in the tank wall while injecting more melted poly into it so that it all runs together and hardens into a into one blended mass. It looks as if there's some kind of filler between the fitting and the tank wall, but in fact, it's all a mass of melted poly. Spin welding and heat welding require a little skill, 'cuz if you don't melt enough of the fitting and tank wall, there isn't enough melted material for the weld to hold...and if you melt too much, it's like trying to fill the Grand Canyon with a hot glue gun. So it's not something I'd recommend trying at home. The simplest and most effective solution for a DIY retrofit is a rubber gasket or o-ring.
