Turns out, there were many pinhole and larger holes. Investigating the tank internals with a snake camera (that has no still image functionality, unfortunately), it's obvious the inside of the tank has been treated and coated. The lowest part of the tank (when normally installed) still shows evidence of rust-though and new holes developing through the tank coating, though not super terrible.
I'm turning to you for this... Is it possible to re-treat and re-coat a tank? I can't imagine the phosphoric acid treatment interacts with a chemical coating they way it would with bare metal and rust.
Is there an alternate approach to pre-treating an already-coated tank prior to re-coating?