1) Get the sick fish into a quarantine tank or storage container with 1 tsp/gallon aquarium salt.
2) Do an 80% water change on the main tank.
3) If the salt does not help in 24 hours, start an antibiotic like Kanaplex. It had antibacterial and anti fungal properties. Melafix may not have enough muscle. I would also treat with Metro+ in this case at the same time.
Keep both tanks free of ammonia and nitrite. Use an API liquid test kit.
Bringing new fish in can bring all kinds of illness. It's best to quarantine new fish and plants for thee weeks minimum to ensure health before you infect a whole tank. Fish get picked on when they are weak (usually due to illness).
Commons should not be with fancies. This causes stress. They need an absolute minimum of 20 gallons each and double filtration. Without this, the water will be very hard to keep pristine and the cycle may not balance. If you are cycling the tank with fish, they have no choice but to suffer through it. Keep ammonia and nitrite under 1ppm if it is cycling. Don't put the sick fish in, even if he gets better, until the tank is cycled completely (0 ammonia/nitrite, any level of nitrate).
Good luck!