The return flow from the sump was just a trickle. Tried different pump speeds but no change.
Did the return tube plug? It looked OK. The braided nylon tubing is about an inch wide, clear, and some algae at a glance but nothing obvious that could plug it.
Did the return pump fail? Pulled it out of the sump, placed it in a sink of water and started it up. The pump is working!
It must be the return line. I needed a hair dryer to disconnect from the barb. Blew threw it and sure enough some resistance, but what and where.
About 6 inches from one, some bubble algae had build up and clogged the tube. Removed with a screw driver. Hooked up a garden hose to blast through the tubing. No luck!
Placed the tubing on a hard surface and stepped on the tubing along the length where there were any dark spots. That broke the bubble algae free. Cleared the tube with high pressure from the garden hose.
Hooked up the return line and this time with more lube.
Problem solved.