Hi and welcome to the forum
Can we get a picture of the entire aquarium?
Do you have any pictures of the fish under normal white light?
Do you normally have pink/ purple light on the tank, or is it normally white?
If you have live plants, they need a combination of red and blue light. Fish need a combination of light too and generally don't do as well when under just pink or just blue light.
Fin rot is usually caused by poor water quality that damages the skin and tissue, and this allows harmful bacteria in to infect the fish. The best treatment for fin rot is big regular water changes (75% every day for 1-2 weeks) and salt.
As Aiken said, don't add medications unless you know what the problem is. Most fish health issues are caused by poor water quality and adding toxic chemicals to an aquarium with poor water quality can kill everything in the tank. Even if the water quality is good, adding poisonous chemicals to an aquarium will stress healthy fish.
Most aquarium fish medications are made of poisonous substances and treating the fish is a balancing act where you add enough chemical to kill the harmful pathogens, but don't add too much and kill the fish.
If you have concerns about fish health, the first thing to do is wipe the inside of the glass down, clean the filter, and do a big water change and gravel clean the substrate every day until the problem is identified. This removes a lot of disease organisms and dilutes nutrients in the water, thus giving the fish more chance of recovering and fighting off any disease/s they might have. It also buys you time to find out what is going on.
How long has the tank been set up for?
How long have you had the other fish for?
Did you rearrange the aquarium decor just before adding the new fish?
Have you added anything to the tank in the 2 weeks before this happened?
What is the ammonia, nitrite, nitrate and pH in numbers?
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
Do you have any live plants in the tank?
Discus need lots of plant matter in their diet. See the following link for more information about their dietary requirements.
If you have older bigger discus in the tank, and you added new younger fish, the older ones could have attacked the new ones when you left. This might be from lack of food, whereby the older more established fish tried to drive the newcomers out of the area because there was not enough food for them all when you went away.
If you have a mature pair they might have attacked the new fish because they went into the pair's territory.