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If the "catfish" is the one in your signature photo, I hope you have a 5-foot tank and 5+ more of the same type... That looks like a Synodontis decora, possibly a hybrid variation but hard to tell, which will reach ~30cm and are highly social for a Synodontis.
Until you tell us what the "gourami", "danio" and "catfish" are, the minimum temp I would go for is 22/23C. But as said above, if we are talking about Zebra/Leopard Danios then they need a temperate tank, which will be too cold for the others at 18-20C for most of the year.
What type of danio and catfish?
Zebra/Leopard Danios like very cool water, (less than 20C), Celestial Pearl Danios like more usual tropical temps of 22-26C, and Pearl Danios are kinda in between.