If it's Saprolegnia fungus like I'm think then it likely came on him following a injury. Possibly a bit of a fight with the Aro.
If he has what looks like white fungus patches on him then you needs anti fungal treatments along with vitamin C rich food. It probably shouldn't be mixed with aquarium salt so you may wanna give the salt some time and if there is no improvement do some water changes and bring in a half dose of anti-fungal.
How long of an injury because I have never seen the aro have any aggression but about a month ago he was with African cichlida and the tore him up do you think it could be from that long ago??
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As long as the injury hadn't fully healed then he could have been infected at any time. In other words the injury could have happened last night or a month ago. Although the Aro hadn't shown aggression they still could have had a fight over night while no one was watching the tank. (could of been a 1 time thing over a hiding place or a left over piece of food)
Fin rot. Usually caused by fin nipping from other fish .. Once they're nipped at the bacteria attacks breaking down the edges of the fins and sometimes showing red on the fins which is from inflammation. Anti-fungal treatments can work but so can salt which you're already doing (I think).
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