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Hi,

I would not recommend only one fish per tank, except when it's really not possible (fin-eating fish, scale-eating fish, etc.).
Nearly every fish I had completely solitary (only temporary) did react not too good (they became lethargic and shy).
In my eyes most life-forms (especially the not too un-intelligent ones) are not made to life their life as the only lifeform in their environment. Especially with cichlids it's not the best idea, many of their behaviour correlates with other fish and you won't see it with a loner fish.

A good example is my old obscura-snakehead. He does not really like most other fish, very only few other fish are allowed to be in it's direct near. If no other fish is present he'll become very shy, lethargic and will stop eating in a few weeks.

greets

Peter