It's hard to say without a picture if there is another underlying problem other then fin biting.
Going see through could be a number of things as well from new fin growth to lack of enough nutrition.
Are the red spots on the edges of the fins or in the middle?
Are the edges of the tail black to very dark red? Can you tell if it is shredding/flaking off or are there small C/U shapes on the edges?
Stress coat alone won't stop fish from tail biting- it's a bad habit they pick up from stress or boredom (not having enough in their tank) usually. And it's frustrating because there really isn't much, if anything, we can do to stop them. All we can do is make sure the water changes are on schedule so rot doesn't develop.
You can try giving him more attention, rearranging the tank, adding in new things, change his diet.. I can't really think of anything more that unfortunately.
Here is a little fun thread
made by a member here for tailbiters.. just so you know we feel your pain and frustration, worry.
*Edit- when you talked about him before, did you mention whether you have a filter? If so, have you baffled it? Sometimes their tails can cause a lot of drag and be heavy, so they will bite it off.. what tail type is he?