At the moment, the aquarium is heavily stocked and I'm assuming the water quality has gone down. The fish include a broad range of Tiger Barbs, a Flagtail Prochilodus, Clowns, many Yoyos, Kuhlis, etc. All of the fish are still juveniles. I do 40% water changes every Wednesday and Sunday so that seems to keep it relatively stable (I hope). I mainly feed my fish Spectrum Thera +A and live/frozen Black/Blood Worms.
Recently one of my Tiger Barbs have developed a rather intense case Fin Rot. I've read mass water changes can cure this but I have yet to see any results.
My question is whether or not adding aquarium salt would help remedy the fin rot and further prevent my already heavily stocked aquarium from developing other diseases to the fish. (any other methods would also be appreciated).
This would only be short term as I will be upgrading within the next couple of months...
Some pictures of the aquarium:



