Sometimes the gunk on the inside of the hoses breaks free and gets pushed into the tank. This happens more often with the newer units where the inside of the plastic is still very smooth. Less gunk breaks free over time, so you don't have to keep cleaning them out all the time.
One cheap diy way of cleaning the inside of the hoses out is to use a wire hanger, bent out straight, a long piece of strong string, and a cut up rag. bend a small loop in the end of the hanger after you strighten it, tie the string to the loop, and use the wire hanger to feed the string all the way though the pipe. Once you've threaded the pipe with a long piece of string, tie your rag into the string at the midpoint, and floss out the gunk in your tube. Just make sure it's strong string so it does'nt break, and it's at least three times the length of the tube, so you can take the rag back and forth without having to re-thread it...
Heh, do this outside if possible, that gunk likes to miss the sink and wind up on the floor!
