I'm going to bite. There are so many materials labelled as silver solder. In the electronics industry it is sometimes use to include tin/silver alloys with a melting point lower than tin but often still higher than eutectic tin/lead. It will often have small amounts of copper.
Brazing rods can include silver but these are devilishly expensive. Standard brazing rods less so. Propane can get a lump of steel up to temperature but has to be a good lamp.
I thought the normal way to repair steel was to use tin/lead bars, with a composition a little away from eutectic so to enter a paste stage?
Water and tanks sound a bad combination for future rust. Another way is to fill the tank with water and displace the fuel. I guess that become a disposal issue for a business.
Sometimes used in boats and other places where heat is applied and to prevent fires from starting.
More painfully I've come across tanks that are £450 from a third party where second hand is still in the £200 region.