sick or nipped fish?

[tengodown]

Over view. I have a 100 gallon brackish tank with 2 archers 3 scats
3 columbian sharks(2 big 1 small) 1 snow flake eel and 1 molly. ok so my columbians have gotten sick/nipped which kind of looks like fin rot or could be nipping. the whites on the side fins tail and under belly fin have been mostly removed. Its been hard to decide if it was fin rot or not. my smallest shark has gotten a red under jaw and a large blister in the right corner of his mouth. this has all happened with in a week or so. my heater did just go out and has been replaced with a heater that can keep it about 79 was 82 before.

any advice? ideas?

P.S. waiting to dose fin rot till new heater arrives.

 

[lindenthaler.9]

What size are the sharks now and what's the salinity? They usually develop these conditions in freshwater so your salinity might be too low for them.
As adults they need full strength saltwater, brackish isn't enough for them.
I think your eel might be having a go at them. I have Morays as well that are brackish and looked into the possibility of keeping them together. It can work for so long, until the eels decide it's not. I'd separate the sharks, give them their own tank. (Preferably bigger if you can) I'm sure you know of their potential size. That will also allow you to give them full saltwater.
The Snowflake should probably be raised to full salt as well. I know my species that are brackish eventually require it.

 

[lindenthaler.9]

Also, what's your substrate? The sharks and eel should both have sand due to their lack of scales. If it's a rough substrate that could be causing some more damage.
I don't know if the heater going would cause this, but you can look into effects of them being kept in cold water.
It's also a possibility your Scats have been after them. I don't think Archers would be responsible.. whether it's any of their tank mates or not I'd still separate them. They have a better chance of recovery and long term health in their own tank.