You can clean the tank using a power washer and just about any good detergent (Dawn dish washing liquid is good). Then after you've replaced the tank and reconnected the plumbing, finish the job by recommissioning the entire system, 'cuz the plumbing is actually the source of most foul water, because the molds, mildew, fungi and bacteria which cause it thrive in damp dark places, not under water.
I can send you the complete instructions for doing that if you want to send me a PM that includes your email address (no way to attach anything to a PM).
--Peggie