Doesn't look cross threaded to me - incredibly hard to do with copper/brass fittings anyhow. Tape on the threads won't help because the nut end isn't sealed (I would have thought this would be intuitively obvious). I always use a wrench to torque down the nut and have never damaged anything (you want the nylon washer to compress slightly). You will want to check the nylon washer to make sure it isn't cracked.
Sometimes the CO2 tank places will give you black fibrous washers .... which I don't find work well at all.
Sometimes the CO2 tank places will give you black fibrous washers .... which I don't find work well at all.