Originally Posted by Jayy
Well I did a water change last night, ( thank God) ,but my four lil fish died. I'm not sure why, but I hope my water's ok. I might get check out next week.
Could have been the neglect caused high nitrates and it finally got them, some fish are more sensitive than others. But another perhaps more likely reason is doing a water change that altered the chemistry too much.
When water changes are not done, organics build. Nitrates usually rise in response, but pH may also decline. This depends upon the buffering capacity of the KH in the water. If for instance the pH becomes acidic, then all the ammonia being produced by fish and the breakdown of the additional organics will be relatively harmless as ammonium. If a water change is then done with tap water that is basic (pH above 7) and the pH in the tank should rise above 7 because of the water change, the ammonium immediately changes back to ammonia. Result, dead fish.
I know things sometimes get in the way, but it still must be said that regular partial water changes must be done. But if they can't be, then do very small changes over a period of days to bring the tank back into balance more slowly. Any significant changes in water chemistry, whether pH, nitrates, or whatever, can be fatal as you've seen.
Hope this will help at least in future, and for others who may find themselves in a similar situation.