Ramrod suggests exactly what I did, when I found myself in the same situation you're in.
The radiator guy here in LST professionally cleaned out my 240 tank and treated it, for $350.
It was either that, or order a new one from Skandix for about $650 AUD + freight.
Pulling the tank isn't particularly difficult, just cumbersome - it's seven 14mm bolts and some faffing about with the clamp seal and inlet pipe. You can do it solo but two people can do it very quickly.