How long have you had them? What other fish are in with them? Any new additions? Better yet...
Answer these Qs to ensure we have as much information as possible about your ailing fish and its environment:
1~What type of fish is afflicted? In addition, please describe what is wrong with the fish to the best of your ability (i.e. cotton like growth, bloated, etc.).
2~What are your tank parameters (ammonia, nitrites, nitrates, temp, pH)? Please give exact values.
3~ How large is the tank? How long has the tank been set up?
4~What type of filtration are you using? Please give the name and number (i.e. Fluval 304) and amount of gph
5~How many fish are in the tank? What kinds of fish are they and what are their current sizes?
6~When is the last time you did a water change and vacuum the gravel? How often do you do this? How much water do you remove at a time?
7~How long have you had the fish? If the fish is new, how did you acclimate it/them?
8~Have you added anything new to the tank--decor, new dechlorinator, new substrate, etc.?
9~What kind of food have you been feeding your fish, have you changed their diet recently?
I had an oto die of similar symptoms. It was probably columnaris that caused that one and it can spread rapidly. Regardless of what your answers are to the above, I would do a water change asap and keep an eye on the others. If you suspect anyone else of being sick, quarantine them if you can.