Planted Tank Guru
Join Date: Jul 2010
Location: Pflugerville, Tx
Sounds like a blockage of some sort. One that I find easy to miss is out of sight. I've had one of the "macaroni" pieces get into the elbow at the bottom and hide. Forcing water back through the elbow cleared it for me. Also it is possible that the tubing is getting full of gunk and needs cleaning. I take mine off when needed and run a rag on a string through it. I tie a small wieght (screw,nut,etc) on the string to drop it through and have a pardner "saw" the rag back and forth to clean any gunk out.
Very unlikely the filter is worn out to the point of slow flow. They will get way noisy first IME.
One item often ignored is cleaning the impeller and well where it fits. I clean the well with a Q-tip to make sure the head is all cleared.