what killed my fish?!

while I was away, my mom was caring for my fish. It was hot out so she keeps the AC on in my room,
the heater was unplugged, and theres NO WAY it could've gotten cold enough to kill them. and the temperature got to 83 degrees.
Thats the only abnormality.

I brought my water to be tester at a store after testing it multiple times at home...
nothing alarming. everything was perfect. which I was relieved about, but still confused as to why this happened.

I haven't had any new members added to my tank for awhile, I added new corals in feb. or march. I believe. Thats the newest.

The tank is always covered over because I've had a fish or two jump out before, we're careful about covering it.


I have been adding reef plus concentrated vit. and amino acid supplement for the corals, but the guy at the store didn't seem concerned about that.




Any ideas?!

 

maybe lack of oxygen?

 

Ive had fish die for no good reason in my tank for the first time. I feel for you bud.

 

It may always be a mystery but if I were you I would google or do a search on the DIY forum for screen covers, net covers, eggcrate covers. Ideally you want as much surface area open to the air to allow for optimum gas exchange. Also another thing you may consider doing is setting your heater to the temp you want and leave it plugged in. It won't come on as long as the temp doesn't drop to the set point BUT if the water temp does drop and it's NOT plugged in it's just a worthless tube in your tank. It takes less time for water temps to fall into the danger zone than you think. Just my thoughts.