Fish died when I stepped out for 20 minutes

[ThomasG07]

I have a 10 gallon tank that I bought on Thursday. I let it stabilize for 48 hours and then I put 5 hardy Red Minor Tetras in it. They were doing well for a about 8 hours and then I ran out for 20 minutes and when I came back one was upside down at the bottom of the tank between the heater and the glass. Ammonia low and nitrites and nitrates at 0. HELP!!!

 

[Thyra]

Welcome to the forum! A tank does not "stabilize" just because you put water in and let it sit. You really need to read about the nitrogen cycle. Any amount of ammonia or nitrites are very harmful to fish. There are several ways to cycle a tank, but because you already have fish, you will have to do daily - about 50% water changes - to keep the ammonia and nitrites at zero. When the beneficial bacteria kick in you will start to see some nitrates which you will want to keep at around 20. Are you using tap water that has been chlorinated? If so, you also need a dechlorinator.