Hi Larry, u r no doubt right about the fat, but in the case i referred to it was a toilet block in a tourist park, which was open periodically - high volume when going, then nothing for several monthly periods - no fat or oil, maybe its our heat too, but top crust would still form hard and airtight as there was no movement in primary tank. In the other system hooked to the kitchen, (which rules although strict about fat & oil not entering) i knew some was getting in, but cos the caretakers residence was hooked into same system & was used daily, still never got an airtight crust. re drains blocking, we use course sand filter alongside when first built, & have 2 drains which operate alternatively, by diverting one off for 4 months in summer, the sand dries out completely and then will start reabsorbing, then change over. both systems in place for 22 years - none ever pumped, but crust broken each year. it was the periodic use of the 50 gal system that i thought would still get a crust if not used regularly - but hey u know the climate & environment better than me. i just like to be able to get into any system without opening pipe lines:thumbs up:cheers OzGH