Whitish edge on dorsal fin

I have one glo lite tetra that is showing something on the dorsal fin. It looks to be nipped a bit but I've never seen any nippy behavior among the tetras. There is also a somewhat whitish edge on the fin, as can be seen in the photo. Any ideas, fin rot or just a nipped fin on the mend? The other tetras all seem fine. I noticed this about a week ago and the particular fish was sluggish and stand offish. However, I started changing the water, about 40% or so every couple of days and he seems to have repsonded and the fin has grown back a bit, it is just the whitish edge that concerns me.

Thanks!

John

Thanks for the replies. As I said, I have been doing water changes every couple of days and the fish seems to be doing better. I'm going to stick with just the clean water for the moment as I've a bit averse to adding meds to the tank until absolutely necessary. Since I know the cories didn't take a chunk out of him, must be one of his (or her) tetra buddies.

John

It's currently stocked with glolites, Sterbai cories and male Endler's, so I'd imagine it has to be one of the other tetras who is doing the fin nipping.

John