I polish my fuel and don't generally have fuel problems. But, on my sailboat, I did have a problem with a clogged pick up tube. I polished the fuel and didn't find anything that looked like it would clog the tube. So, it might still be in there. The pick up tube is 1/4 inch, and this seems small to me, easy to clog, and I've heard of others having this problem, generally from people that don't polish their fuel. I'm assuming that there is a diameter restriction of the tube based on the volume of the pump due to the viscosity of the fuel? If so, I can't enlarge the tube since the sailboat uses very little fuel and pumps small amounts. I've never had water or clogged filter problems on the sailboat or trawler. Another question, on my trawler the two fuel tanks originally were connected and fed a filter system. Both tank's pick-up tubes are from the top of the tanks and joined together feeding the filters at a lower level. There was no cut-off valves. When the boat was at rest I would find more fuel in one tank than the other and the boat listing slightly. After cranking the single engine the fuel levels would equalize. I'm not sure I understand the siphoning, atmospheric pressure equalization, etc. that was going on, but it worked. The engine return line is to one tank, but when running the tanks would always equalize. It seems logical that if the tank feeds were from the bottom that they would equalize, so I'm assuming there is no difference with the feeds from the top?