It does look a lot like some kind of healing bruise, nothing worse

So you just keep taking care of your fish! Like Goldiegirl says, as long as they eat and act perky, they're usually perfectly healthy

I didn't catch if you had a test kit or not, but if you're worried, you could take a sample of your water to a pet shop and ask them to test it for you! If the parameters are as follows: Ammonia 0, Nitrite 0, Nitrate 10-30 and pH around 6.5-8.5, all you need to do is a 50% water change
