Leaky CO's

During a 2-story standing water test of my DWV I notice that some of the CO's in basement had the smallest leaks.... I thought they were already pretty darn tight and I'm a little concerned about overtightening them. Are they just not tight enough or was I supposed to use teflon tape or something around the threads? Or is there just too much pressure w/ the water test and since the system is usually not pressurized it is nothing to worry about?

Thanks!

You WERE supposed to use a thread sealant of some kind, but they would probably still leak. There are several possibilities to stop the leak, either a thicker joint compound, additional wraps of Teflon tape, or a rubber cleanout cap clamped over the fitting's outlet, (and over the plug at the same time).