There could be a few parasites that could be doing this. Best option would be to ask your local aquarium store, they usually have tablets or solution to get rid of most common parasites.
Otherwise if you have lost all you fish, start from scratch sit your filter mediums in really hot water, throughly empty out you tank, put you gravel or substrate in a bucket and fill it with boiling water to kill any nasties hiding out.
And just keep you tank clean (water changes ect)
And when you introduce new fish to the tank maybe quarentine them for a week in another tank before putting them in.
(Some people say you can use bleach to clean you aquarium stuff but I prefer the All Naturale Approach ie. Boiling water)
Good Luck