Water quality is something you should look at first. If your tank has been running for a while and you know your cycle is healthy (no ammonia or nitrites) then test your nitrates. Nitrates are the last product in the "cycle" where bacteria in the filter process the waste into less toxic nitrate (with an 'a').
If you're not absolutely sure test for ammonia, nitrites, and nitrates.
Regular water changes are the method by which the steady rise in nitrates can be diluted/reduced to safe levels. Topping off doesn't remove nitrates, so you have to actually remove a portion of water, say 50% at a time until nitrates are at safe levels. Then you can do water changes when needed.
You can either buy a water test kit that tests ammonia, nitrites, nitrates, and pH, or if you think the cycle is healthy just buy a nitrate test kit.
OR. Bring some of the tank water to an aquarium store and have them test it. Almost all aquarium stores including Petco, Petsmart, etc do it for free.
I would also get a large bottle of Melafix. This is a natural product made from tea tree oil and it helps with minor infections and healing. It's pretty hard to over dose with this stuff, and it'll make any device creating bubbles to suddenly make a blizard of tiny bubbles which are harmless to the fish.
I wouldn't worry about it being contagious. I wonder if it could have occured during a fight with another cichlid though, but never the less, nitrates below 20 or less is good and the Melafix is a great product and won't interfere with your benneficial bacteria.
Good luck, and let us know if we helped.