Most of the time when fish get sick for no apparent reason, the root cause is one or more water quality issues. First, make sure temperature and pH are appropriate for the species you are keeping, then test for toxic nitrogen compounds; ammonia and nitrite (NO2) should measure zero, and nitrate (NO3) should be well under 40 ppm.
Inappropriate water parameters and/or uncontrolled toxic nitrogen compounds put considerable stress on the fish and thereby weaken their immune systems. Since even well kept aquariums contain dormant forms of diseases and parasites, a weakened immune system makes fish vulnerable to all these things. Often, a disease or parasite infestation is not apparent until it is well established and thus difficult or impossible to treat.
If toxic nitrogen compounds are not at the above recommended levels, start changing about 25 percent of the water every other day until they are. During partial water changes, syphon any debris off the substrate surface. Adjust the replacement water temperature to roughly that of the aquarium, and treat it with a good tap water conditioner.
After toxic nitrogen compounds are controlled, change about 25 percent of the water every week, avoid overcrowding, and feed no more than the fish actually eat in a couple of minutes. During routine partial water changes, syphon debris off the substrate surface, but deep clean only about 1/3 of it an any week. The beneficial bacteria associated with the nitrogen cycle live primarily in the substrate and filter elements, and the cycle will stall if too much bacteria is removed.
Similarly, change filter elements infrequently, never all at once and never in the same week the substrate receives a partial deep cleaning. If a filter element gets clogged, gently swish it in used aquarium water to remove debris. A filter element needs changing only when it’s in shreds. To help keep the nitrogen cycle working, put a few shreds of the old filter element with the new.