Just wiped out half my malawi stock!!!

[malawi500]

I have/had a 80g tank of about 25 malawi cichlids. I just returned home to find 11 of them floating at the top!!!

Water temp seemed fine, clarity looked good. I tested the water as follows:
Ammonia 0
Nitrite 0
Nitrate 10-20

The only thing I have done differently was put a teaspoon of kelp in the tank yesterday. Would this have killed them or should I suspect a dodgy heater (Rena)?

 

[ecoli73]

What is the temperature of your tank?

 

[anarekist]

maybe there was a battle? or lack of oxygen.

 

[malawi500]

26deg, I swapped my heater over also to my virtually brand new one and that registered it as 26 also.

Yesterday the kelp did cover the surface for a bit but was clear by this morning.

I have also just carried out a 25% water change to be on the safe side. Rest of the fish are fine so I guess they died earlier on in the day either through overheating or lack of oxygen as you suggest.