Quote:
Originally Posted by Stitchawl
3. The tank is only set up for a couple of days ...
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This is the first thing I noticed - every tank that I've ever set up I've waited at least 2 weeks before adding any fish. A couple of days doesn't allow the environment to stabilise the nitrogen cycle.
You also stated that you have changed the filter material several times. The filter relies on 'good' bacteria colonies to neutralise ammonia so the more often you change the filter media, the more likely you are to have raised ammonia levels. Ammonia will kill fish at levels too low to be detected by home kits so this is a possibility.
The third option is that one of the first fish to die was carrying a disease that is now lurking in the gravel and/or plants. Complete sterilisation of the tank and gravel and starting again would be worth doing if nothing else helps.
I hope you solve it soon, it is so frustrating (and expensive!) when this happens.