Water tank conversion to diesel

[phil23]

We have a water tank, grp built into hull, that has been in use for approximately 15 years which I would like to convert to a diesel fuel tank. Is this considered to be feasible and if so what would be required to render it suitable for carrying fuel.
Are there flushing solutions for cleaning we could use as currently access ino the tank is somewhat limited without cutting open the side

[atoll]

can't see a problem as long as it doen't leak
you will need ti fit a return line to tee in with the existing return line with a 3 way valve.
alge from inside the tank falling off might cause problems for the filters,so an inspection hatch and steam clean might be a solution.