Will rust
It will rust eventuallly
I washed it well with vinegar, then painted it with some sort of epoxy paint. After about 5 years, the paint started chipping off. Probably should have used a primer coat! Eventually parts above water were rusting slightly.
At the same time, I had a lot of fish deaths that I feared might be due to the paint chips being swallowed and the rust, I have since tracked that problem down to the ancient flake food I was still using (bought 10 lbs in 1992? and was still using it in in 2002) Anyhow I sealed the shell with EPDM liner ($38 online) since I doubted I could depend on the paint holding up since no matter how I scrubbed and pressure washed, more chips came off. So, I painted it to seal the rust and then covered it anyhow.
It is OK for fish, according to most people. I was never certain, but for that price vs the cost of a black shell from a pond store (then about $800) I was Ok with taking the chance.
If you can get it painted correctly, and not have it chip when the goldfish tear the algae off to eat, then I guess it'd be OK. If you can live with the eventual rust, and don't really care about what happens 5 or 10 years down the road, then it is a reasonable solution.
The pond in the science garden at the elementary school is bare metal, I figure in 5 years (or less) someone will vandalize the pond and blow it up with a cherry bomb or gun, anyhow.