Before You Replace Your O-ring....

....you might want to check this.

I thought my 12FSS fitting was the problem because I still had the leak after changing the O-ring. I thought I had it at first but it still leaked on the next fill.

I then checked the male fitting with my loupe and this is what I found. I don't know if it came this way because the tank is 6 months old and was fine on the first fill. After the second fill I stated having the problem. It may be the place I'm having the tank filled but I'm not sure. It's a commercial place that fills firehouse tanks so I think they would be on top of it.

Another thing is that there are so many divots in the fitting it's hard to believe having it filled just two times would do that much damage. Can my 12FSS fitting be damaged and compromising the male fitting ?

However, what I'll do is replace the male fitting and the next time I have it filled I'll check my fitting with the loupe and see if there is any damage.