It could be due to fighting, he could have caught it on something in the tank, it could be a vitamin deficiency- hard to say but fin splits heal very quickly and as long as the water is clean and there's no fin rot and splits don't keep on happening then there's usually nothing to worry about
this is a fair;y extreme example. What you are looking for is white bits around the edge of the fin, especially where it is split. It will gradually eat away at the fins so if you see them getting shorter then it's fin rot. Also the fish will look a bit down in the dumps.
No parts seem to have disappeared but now looking v carefully at the rest of the fish it does look as if a few of them have splits. Have read that it can be from old food and stress of being moved.
1. I am using some flake food that i inherited with a 2nd hand tank - could be too old so I'll get some fresh stuff.
2. We have recently introduced some nothobranchius guntherei and they are absolute buggers for getting into the filter holder bit (if you know what I mean). I am therefore regularly having to open the casing, move things around and fish around for them. It's highly stressful for me so I can't imagine what it must be like for their tankfellows! Thankfully we've had 2 days without a need for a rummage so perhaps they're learning.
Really don't want to lose my gorgeous corys so should i treat for fin rot in case it is early stages? Hmmm...don't like idea of them being ill.
Yes, food does lose some of its nutrients over time. It's worth supplementing with some veg and/or live food (depending on fish species) as well.
Do you mean the filter clip, with the sucker bits on? I had some peacock gobies that always used to use that for laying their eggs, I ended up getting a spare so I could rotate them. Are they really getting stuck or just using it for a hiding/spawning place (no idea about killi spawning).
If you want to you can add some melafix, it might help speed up the healing.
It's a Juwel tank and the heater and filter (sponges and actual mechanical unit) are housed in a plastic 'box' which only has a partial lid because it has to have space for the cables to come out. It's actually above the water line, but the nothos are known to be good jumpers - don't know if they're getting in that way. Other option is that they're entering through the slits where the water is pulled in. Just don't know. Apparently they do like hiding...but why not use the plants?!?! Thankfully they aren't the ones with the 'split fins'.
Anyway, I may get some of the melafix to see if that helps. So you don't reckon I should get antibiotics yet?
Nah, very doubtful you will need anything hardcore for just a split fin. I used to have fancy goldfish and there was carnage when they spawned, there were split fins and missing scales... they always healed up pretty quickly. They were very pampered fish