Change water .
Change 3-4 gallons a day for a couple days then up your water changes to 50% once a week.
it is a wound from thrashing around in a crowded tank) may become infected if the water is not kept clean.
Having not changed water much at all is not good for the fish so start slow like I mentioned.
Make sure you dechlorinate your water and try to get it close to tank tempature.
Either mela fix or pima fix may help?
I don't really take much stock in either of them or know the difference ,but one of them is made to help wounds heal.
Neither of them are any kind of med that would cure anything.
More like bactine/Neosporin IMO
and may help the wound heal faster or better.
A API nitrate test kit would be all you need to test your water as I would imagine a 7 year old aquarium would be cycled(no ammonia or nitrites).
But the test kit is not a "must have" IMO
for you at this point.
Just start to change water regulary after the first week of slow smaller waterchanges.
This is for fish safety as if you have not changed water in a long time things like ph,gh
and nitrates are going to be all out of whack compared to your source water.
Hope he gets better.
that is an iridescent shark,as Caleb mentioned.