Can't offer much advice sorry. But I had a tank sitting around for a year or so. I drained it but never cleaned it.
Anyhows.. I've stuffed 2 EFI pumps since from dirt/crap being sucked up.
Just brought a new pump but reluctant to use it until I clean the tank or install a filter before the prime pump maybe (which is external on my setup).
Just put some petrol in it and swish it around! Do it 2 or so times! If you want a good job done take it to a radiator shop and get them to flush it with acid!