I cleaned mine out this year using a similar method described by "aristocraft" and it worked pretty well. The problem I found is that the tank appears to have internal baffles so it is difficult to determine if you got er done or not. I installed a clear - cleanable inline filter so I can see if there is any crud getting past the pump. So far so good. Just completed a nice long cruise on Wedowee and she is running great.
luva63 - I am working on replacing the below deck 30 gallon belly tank in my 89 19' Capri Cuddy. I am having a hard time finding something that will fit. Your solution interests me. Do you have just the one tank? Did it affect your stability at all since it's not centered on the hull?