Sorry to hear but this is sadly a normal part of a hobby where one keeps live creatures.
Things like these don't happen for no reason. Usually it's one of the two:
-- either a new fish, new plant, live food (a wildlife visitor of land or air if talking about an outside pond) introduced a pathogen
-- or the fishes' immune system has been significantly compromised / suppressed by some kind of constant, long term, low level stress - toxins NH3, NO2, NO3, light, bullying or predation attempts, low oxygen (too little agitation, too warm water, too much algae and plants rob water of oxygen badly at night time), contaminants like heavy metals, lubricants, detergents, cosmetics/perfumes, oils, grease, cleaners, sprays, wrong water hardness, salinity, pH, temp, large swings in water parameters, left-over tap water disinfectants, etc.
Even if you cannot pin point the reason now, investigating this thoroughly now will pay rich dividends in the future when things go astray again. And they will. They always do.