Used Tractor Values

I seem to have mold in my overhead fuel storage tank and I am constantly having to change fuel filters on my tractors. I have used Sea-Foam for awhile, to no avail. Any help would be appreciated

Our brand new craftsman lawn tractor has been used twice. And each time, it has backfired once (so twice total, though he’s still out there using it so I’ll update with more info when he comes in). What causes them to backfire? I would definitely assume this is a bad thing? It’s a manual, not automatic.