How did you unblock it?
once unblocked? anything "rattling" around in there?
If something man-made, I would think you could "shake" it out. If organic - I would think that a small wire would have it out to where you could shake it out???
I first tried to pressurize the tank. I used a blow gun and a piece of plastic over the filler neck. It wasn't a perfect seal but you could see the tank swell. No luck blowing it out through the outlet fitting. I then put the air gun into the outlet, blew into the tank and it cleared the blockage with very little effort. I then drained three gallons of fuel through that fitting with no issues. The last three quarts in clear jars. All three gallons through a filter. Nothing ended up in the filter and the three quarts were clean.
After I drained the three gallons, there was I would guess between one and two quarts left, I shook the tank to mix up whatever may have been in there and poured it out. I used two of the three quarts to do this again two more times. I never heard anything rattling around.
Today I took a white rag attached to a stick and rubbed it around wherever I could reach and it came out clean. I have no idea what blocked the outlet. Whatever it was it got worse over time. The length of time the tractor ran got shorter. So I am going to treat for algae and slime and hope for the best. It is very tempting to spend the $250 for a new tank and guarantee no issues. I spent 20% of that to save a 12 +\- year old tank and when you factor in the four bulkhead fittings in the tank held in with rubber seals it's a crap shoot as to how much longer they'll last. Especially with me working on/around, handling, and jostling the tank. I'm trying the cheap route. We all know how that can go.