If HDPE then it suggests it is a welded tank construction rather than bonded. From what I can read it will not withstand temperatures approaching 80 degrees C.
It can be prone to cracking.
My suggestion would be the magnetic fishing rod even if you had to put water in the tank and slosh it around a bit to get the rust close to the magnet.
Failing that try dilute HCl and wait for that to dissolve the rust before pumping that out and starting a caustic wash followed by lots of water until pH neutral. Stand back when changing from HCl to NaOH though so I’d give it a good rinse before introducing the NaOH.
Not a simple process.

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