How do I stop a leak at the joint? I have a PVC pipe connected to a PVC elbow and the leak is right at the connection! There is a little water pressure in the pipes. I have tried a rubber coupling with clamps, but this just slows down the leak, not stop it! I cannot permanently seal the connection either - I need to periodically unattach the pipes to get inside (since this is part of a filter system for a fish tank).