With such a young tank, my first suspicion is that the water quality isn't as good as it should be and thus will be putting a lot of stress on the fish. With fish stress = poor health and disease.
Check you stats for ammonia, nitrite and nitrate and post the results. Also get pH if you can.
Do you use a water conditioner? Which is it? No/poor water conditioners can allow some nasty chemicals from the tap water to both affect your water quality and directly affect the fish.
A picture would be useful for diagnosis - could be a bacterial infection (such as columnaris) or a fungus, but they might have whitespot as well. All of these are treatable but the treatments are much, much less effective if the water in the tank isn't good enough and poor water quality will make it much harder for the fish to recover and will mean they are more likely to get sick again if they do recover.
Have a read of the info
here, partiularly the info on what "cycling" and how to do it. If the information is new to you, it's really impoartant to pick up where you can and get it sorted.
Although this information may not seem relevant to the diseases your fish may be suffering from, everything in fishkeeping is interrelated and it is important to understand how all the different issues, from water quality to the species of fish you keep, will affect the health of your fish.
