Could the change to lfs water trigger infection (from breeding place water to store water)?
I'm not sure what you mean? Lfs water could be contaminated (sale tanks) breeding place? What's that? The river or a shed near you? Both could be contaminated. All could be very different.
It could be (and most likely is) water issues. Either a change or difference for good or bad at too fast a rate. Really there are so many variables here, dependent on source and treatment prior to adding to a tank, it's a tricky thing.
Sometimes it isn't even your fault, things just go wrong, could be a weaker fish?
As for lfs water? You either get salt (marine strength) or pure (RO/DI) pure alone will definitely cause you problems with freshwater fish, it isn't up to sustaining much life. There are very few fish that would enjoy pure RO/DI water. Some type of remineralisation must take place. Some trust must exist with the store. I'm sure they don't offer neutral balanced water, at least not in England.
It could be a subordinate fish. Perhaps there are insufficient hiding places and the weakest fish consistently invades the territory of the stronger specimens.
@caliban, I've done some trawling online, it seems to be a minefield for temperature on this fish.
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British cichlid association.