My guppy has bite marks on its tail?

I have a 40 L tank with 8 neon tetra, 2 glolite tetra, a small sucker fish and 8 or so guppys (majority are males), laterly Ive noticed that one one of my guppys and only that guppy has been getting bite marks on his tail, his relatively new and has only been in the tank 2-3 weeks more. I've never had this problem before but Im pretty sure its the neons doing it, the problem is I got no idea how to get them to stop. Oh and it started about a day after I started adding a small amount of this stuff to reduce the PH levels of the tank a little each day.