most likely a result of ammonia or acidity problems. get tested and adjust if need be. if you're only new to fish keeping you may be making a common but simple mistake. general maintanence is a simple 20% gravel vac, if you're are cleaning the filter then do so in the water you have removed from the aquarim NOT TAP WATER, the chlorine in tap water with kill the good bacteria which breaks down the waste, once filter is clean (which should only be cleaned when required and not every week, depends on the type of filter and whats in the tank etc) top up tank with TREATED water, hence add your water conditioner to the new water befor adding it to the tank. Simply put if you wash anything in the tank in tap water or ever add untreated (with chlorine) water to the tank, you will kill some if not alot of the good bacteria, tank usually can go abit cloudy after this but the problems start a couple of days later once the amonia and nitrites start to build up, essentially your water goes bad and the fish can die. in some cases small increases in ammonia levels which can result from a loss r lack of bacteria will burn the fish resulting in red spots also the tails may begin to burn. acid watercan do the same, both acidity and ammonia are a problem from overfeeding aswell.
imo use prime for our water conditioner (removes chlorine, chloramone, ammonia and detoxifies nitrites and nitrates) and i use stability for my biobooster its a little more concentrated in an established tank hence you dont use as much. both are made by seachem.
good luck.
