Help. Glowlight danio tail damaged

The fin on the tail of one of my glowlight danios is ripped and damaged. It keeps trying to swim and does for a bit then just kind of gives up and lies on the floor. Il add pictures. I'm wondering if this is fin rot or aggression and fin nipping. My tank is 125L. Stocked with 4 peppered corys, 7 glowlight danios, two zebra danios, one leopard and two pearl danios. I originally had 6 glowlights, 6 pearl and 3 leopard and 3 zebra and 6 corys. I figured I was too well stocked and last year I had some kind of disease that meant losing one danio a month. The only danios I restocked were the glowlights. I recently used fluke solve which is uks version of prazi as I lost a golden zebra danio last month. Water parameters are all fine. Ph 6.8, ammonia 0, nitrite 0 and nitrate below 5. I'm just wondering whether to treat with anti internal bacteria meds I have. Or to leave it and see what happens. I'm considering getting more background plants and if it isn't too well stocked il have to get maybe 2 more zebra danios and 2 more leopard ones to stop the fin nipping. I've never experienced fin rot before if it is that so advice would be grateful

 

If you can set up a small quarantine tank it'll be easier for him to get food.
I'm not sure of it's finrot or nipping but both usually heal with pristine water conditions.
Maybe watch to see if he's getting picked on or if you notice aggression in the rest of the school.

 

I change 30% of the water every 5 or 6 days. Recently added carbon to remove the fluke solve meds so the water should be pristine. Is it worth using some melafix? I've heard it does no harm. I just don't want to lose any others but I'm swayed towards fin nipping seeing as everything was OK a couple of days ago. I'm assuming a fan wouldn't get that bad in two days unless it's been caused by another fish?

 

If you think it's nipping then I think it's important to figure out who the aggressor is. If it's the same fish all the time (I know hard to tell with schooling fish) then it might be behind the other deaths in your tank.
If it's multiple different fish then the ones getting attacked are probably sick with something.

I would do aquarium salt rather than melafix. Unless you have snails/shrimp

 

I have a separate shrimp tank that I housed the snails in while I treated the tank tank prazi. I was going to move the nerites back today as the shrimp tank is only small so they will be able to feast more in the bigger tank. I think its got to be the zebra or leopard. They are females and large and only one of each now. I'm reluctant to treat with salt with having corys. No other fish is affected at all
It's hard to tell but their is a tear right down the middle of the tail fin. It tries to swim but swims slanted up for a minute or two then sits on the bottom for a rest. Watching them I'm wondering if they are spawning/courting. A bigger round bellied is being aggressive and chasing them and getting them to chase back. Also another glowlight has a red spot on its side which looks like it may be a sore. Its only small so I'm guessing it's nipping of some kind. Il have to get lore plants and I'm guessing melafix will help with the sore and the fin to be on the safe side. The fish with the torn fin may not make it but its what I can do to help