The container in question is magnetic, and thus it is most likely galvanized steel, if that makes a difference.

I have used galvanized cow troughs before. They are nice and convenient but they rust away and are a mess to deal with. But to help make them last longer you can coat the inside of the tank with a sealant such as evaporative cooler coating. But once the tar has been allowed to dry completely then the treated tank will need to be filled up and let set for a few days while checking for leaks before putting in test fish and plants to see how they do. From what I have read they should do OK but if not you can empty and refill and try again in case something in the coating affects them.
An oily sheen on the surface of the pool's water can come from a number of sources, often just from old mucky soil with decomposing plant material in it. The easiest way to get rid of it is to flood the pool, presuming your pool has a side that allows for drainage in case of flooding. It is easier than pulling all the potted plants and repotting into fresh substrate!

So, then, I probably want to seal the inside in any case. I wonder where my silicone has gotten to...

You can spray the inside with that rubberized spray in a can.It works great I sealed a crack in a stock tank 4 years ago and it is still holding .This will keep the metal from the water .It costs around 10 dollars a can and it goes a long way .I did the crack on the outside and inside and it has not peeled or leaked and the tank is 300 gallons.I have even moved the tank 3 or 4 times after emptying it