After cruising since last Oct 2015 and having no apparent problems with pump outs. I decided to leave the boat on the Ortega River for winter before heading north next spring. When I pulled into the marina I went first to the fuel dock for a pump out. It had been 3 weeks and two people since the last pump out, I have a large tank. Nothing would pump out. We put a few gallons of fresh water in and pumped out a few gallons of brownish water but nothing else. We tried this a few more times and decided it may be a problem with the pump. They brought down their portable pump. Same thing. They sent a man down to inspect the lines for any blockage and he found nothing. All lines seemed free and clear. After five hours of inspecting the only conclusion arrived at was that the tank has turned to mud and needs to be scooped out. My tank is fiberglass and made into the hull of the boat. There is no way to get into it or see into it to guage the consistency of the contents. There is a cover plate where the tank guage sensors are located. As the inspector explained to me (I did not see this myself as there is not enough room for both of us) the cover plate does not open to the tank itself but to a short channel that leads to the tank. If you remove the cover plate you can't see very much. You can't see into the tank at all but you can judge about how full it is. Anyway the suggestion made to me so far is to cut an access hatch to the tank and bucket it out. I have talked to many boating friends, mechanics and marina owners who I have known and worked with over the years and none have heard of crud hardening in a tank that is being used. Any information would be helpful at this point before we start cutting holes in the boat. It is a Sealand Vacu-Flush system and about six months ago I changed tank treatments from the blue SealandD treatment to the KO Bacterial. Yes I use dissolving TP 90% of what goes down is liquid and macerated. How can it get hard???? Thank you very much for any information.