Help Identifying This Injury

Hi all,
I returned home from work yesterday to find one of my dwarf rainbow with this injury.
I have taken a sample to my local pet shop and it tested fine.
Have one of the other fish attacked her or could she have caught it on a decoration in the tank or something else ?
No other signs of distress and is eating normally and swimming round as usual.

Thanks

Phil

Platys, blue rams and pearl gouramis. Oh and other dwarf rainbow.

could be heater burn

If you search google for dwarf rainbowfish, you'll find lots of references saying that they are prone to ulcers. I researched them before I got some, and almost didn't because of what people were saying.
I do wonder if that's what your fish has.

You could try an antibacterial like myxazin or eSHa 2000. Though some sources suggest that sores like this could be mycobacter, in which case they won't work. But it's worth a try.

The most likely way for it to pass on is if the fish dies and the others eat the body. Keep an eye on her, and if you think she might pass away, separate her so the others can't reach her. If you don't have a hospital tank, one of those fry nets inside the tank will do.