How Did It Die?

[rocknurworld2006]

There is another possibility to explain the white film on the body and fins, along with the sudden death. It could be excess mucous caused by the fish being stressed out. If the fish was put into a quarantine tank without an established filter, any ammonia in the water would stress it out and the fish would produce more mucous to try and protect itself. And being a rift lake cichlid the PH was probably high, above 7.6. This would make the ammonia even worse due to ammonia being really toxic in alkaline water. The higher the PH the more lethal the ammonia is. The combination of ammonia, high PH and a fish recovering from being bashed, would have been too much.
Ammonia also burns the fish and allows entry for disease organisms like fungus and bacteria. This would help Columnaris, fungus & septicemia to get into the fish.

there was no ammonia in any of tanks

 

[Rick A]

For the record, each time I isolate a fish - for whatever reason - I fill the spare tank with water directly from the main tank and put some of the media from one of my main tank filters into my isolation tank filter. I assume this is the best way to mitigate amonia / PH / fish stress problems before starting any treatment.

 

[drobbyb]

there was no ammonia in any of tanks

Actually this isn't true. Ammonia is always present in an aquarium. The nitrifying bacteria just convert it before it has a chance to build up to detectable levels.

 

[rocknurworld2006]

there was no ammonia in any of tanks

Actually this isn't true. Ammonia is always present in an aquarium. The nitrifying bacteria just convert it before it has a chance to build up to detectable levels.

u know what i meant this is just getting picky , and it shouldnt be detecteble on test kits, when u test ur tank or u have a water quality problem