Looks like hole in the head disease to me.
Bad waterquality is often due to the problem.
Keeping your water in pristine condition and feeding a varied diet will help your fish.
Additional Vitamine C couldn't hurt.
How big is your filter and did you test your water?
What are your ammonia-nitrites-nitrates-readings?
How often do you change water and how much?
Getting those test results will help us pinpoint the problem, but I'm pretty sure you wanna start changing 50% of your water on a regular basis, starting now!
Hope you get back to us soon, your fish defenitly needs some help!!!