Two Dead - No Signs

[jasmas]

After a bout of clout which claimed 9 of our Mbuna settled down more than 6 weeks ago, the tank has been thriving, all fish doing well, tank in great shape. Plus we've had fry which are also doing well in growing tank.
 
About 4 weeks ago an Albino started spending time close to the top of the tank, but not gasping occasionally blowing a water bubble and then chasing it. It seemed to be the spot he was keeping. Would be active sometimes in the tank, and feeding like all others. Returning to it's spot. 
 
8 days ago, an Albino was found dead in the morning. No signs what so ever of illness. It didn't seem to be the one close the to surface as it was still there blowing the occasional bubble.
 
Yesterday we've lost another Albino! 6 out of 8 remain. Again no signs. What could be the problem?? 
 
Readings are Ammonia 0, Nitrite 0, Nitrate 20 yesterday before doing our weekly 50% water change. 
 
All help much appreciated.

 

[Asteria]

I know that corys are susceptible to nitrates. 
Are you using a liquid test kit or strips? 
 
If it's the api freshwater master test kit. Then you have to shake both bottles vigorously.  Especially number 2 bottle. Bang it on the floor or something. Then test again for the nitrate. Shake it really well and let it sit for at least 5 mins 

 

[jasmas]

Thanks for reply. The Nitrates are OK, we get almost 10 from the tap. Yes we master the 30sec shake of the 2nd bottle and a minute at the end.
 
We are still baffled, think i'm going to move anyone who is possibly ill to hospital tank and treat. But for what??