We need more info... what size tank? Water stats? Sounds like a bacterial problem to me, but without knowing more about your tank, we would only be guessing. Good morning and welcome!
Thank you..Tank is a small 25litre, only got three black widows now as lost 4 fish last week...heater went wrong as was overheating water...probably why got bacteria...tank is established (around 3/4 yrs old) had water tested twice in last week, nitrate slightly raised but today all normal...been usinf Mxyalin (?) done three 50% water changes in last week.. use decloranated water and Stresszyme.The fish with pop eyeand cloudy eye now looks as had its fins nibbled..I know this happens sometimes when a fish is poorly..any advice most welcome Thank you