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Whatever it is, old water, bad gas that has gelled into varnish, phase separation of an ethanol blended fuel, you're going to have to get the tank professionally cleaned. I wouldn't run that through the engine even with all those filters. I used to live on my trawler in a marina and it wasn't uncommon for people to have a fuel polishing service clean their fuel in the tank but that was diesel. Don'tknow if they do that for gasoline. That "stuff" is probably moving around the bottom of the tank and is picked up when it happens to pass the pickup tube. You could be changing filters for a long time and that could get expensive. If you can't have the tank cleaned or the fuel polished, I would position the boat to make one corner of the tank lower than the rest, remove the access plate and start removing fuel for disposal from that corner until you get clean gas.