could there be a hole or crack in the tank?

Could be.

or maybe a paper clog? what do
you suggest?

Tom

Pump it out.

Throw a bottle of food coloring into the tank, and then fill up the tank with
fresh water. Look around the edges of the tank itself for the appearance of
red, yellow, blue, or green liquid- you'll know that's coming from inside the
tank. Check your hoses, and most especially your hose connections for the tell
tale color. Once you have found the defect, make a note of it. Then pump the
colored water out of the tank, flush out the bilge before the food
coloring "sets" into the glass, and finally proceed to replace or reconnect
your problem.

(Unless somebody has been stepping on the tank to get aournd the engine, the
tank itself is most likely OK)