I've had a diver come in and tell me that inside the stair return the pipe was cracked but he had temporarily plugged it. I ordered a frankenplug, which is ingenious by the way, but noticed the leak continued way below all of the returns. It seems to have stopped where it is now. I haven't run the filter in quite some time. Logic would suggest perhaps a tear in the liner or perhaps a crack in the stairs. Given that it's so low below the returns, I'm going to guess it's not in the return lines including the main drain. What does the TFP group think?
