I just changed it yesterday to the black mesh... no I meant to say 1 tbsp per 5 gallons (so just over a tbsp for the 6 gal he shares). I've had him for about 2 wks. Should I stop the salt and do salt baths if needed?
I don't like doing salt baths... I think it puts too much stress on the fish. What I would do is leave the salt for 1 more week and try to raise the temp.. if he starts to get healed up nicely he probably just bit from stress. After the week do a big water change to get as much of the salt out as possible (of course if your tank is cycled you don't want to do more than 60% at one time).
You might also want to try IAL because you can leave them in full time and they help with stress and promote healing.
IAL is indian almond leaf.. you'll have to order it off ebay bc I don't think there is any place that actually sells it. Its pretty cheap though. I really recommend you get some. They release tannins that tint your water and calm your fish as well as having antibacterial properties.
It seems he is definitely a tail biter. Try keeping the tank lights off for a few days and see if that helps. Look at his tail and make sure it doesn't look like he has an infection of some sort that could be causing his tailbiting. Since you just put the divider in recently wait a week or so to see if he gets over the stress.