Great comments, especially 85 Hokie: My college chemistry didn't mention this stuff. Even organic chem, not mentioned. I've been using the diesel treatment to kill "bugs" and algae. Maybe not effective.
This tractor had been on sales lots for unknown period, before me, New.
Anyhow here is my plan so far. Draining the full tank via the first filter may have caught some crud. I will first cut it open and examine. It might be sufficient for me to stop there.
After getting tank out, assuming filter shows nothing special, I plan to dump in about a quart of diesel along with about 8 (count 'em) small nuts. Then will switch that around by fhysically shaking the tank. Carefully dump via one route to clean large pan, examine it. Try again, and again.
If any stuff sits above the valve (I installed that wayback) blow compressed air in to dislodge any. Repeat agitation with 8 nuts,until no debris appears for a few trys.
Finally will wrap a clean heavy nut with clean intact cloth securely and get to roll around in there. Examine it for stuff.
Hope that does it.