We have setup 2 small systems on our balcony 7 months ago and everything been growing well and healthy. Got 6 goldfish in my tub and usually feed them 3 Tablespoons full of worms daily. This morning I found one dead fish in the tank. Immediately did all tests and everything is at the standard level. pH 7.4/Ammonia 0.25/Nitrate 0.
Deep down I am hoping that the fish passed because of over eating. Cause the other fish seemed to look alright. I went through some of the discussions here and everyone seem to have different scenarios, thought I give it a try to shout out and see if anyone can help shed some light.
Did you test for nitrite and have you done any pH changes lately? Sometimes fish just die, and there isn't really a problem. If you keep losing fish though, something's wrong.