Paradise fish

[Guacamowle]

Now that DS has moved into his own place and taken the snakes with him, I have an empty space in the sitting room that I can finally fill with a much longed for fish!! As a child, we had a pond with goldfish - our garden is not suitable for a pond, and I do not have space for the massive tank goldfish would need, but a friend has a single male paradise fish (gorgeous fish!) in a 60 litre tank and he seems happy. Would love one of these, but a few questions before I go ahead because I want to be sure I am doing the right thing!
1 what is the right tank size for a single paradise fish?
2 is it OK to keep a male paradise fish on his own?
3 my friend doesn't have a heater, and I notice my local fish shop has paradise fish in with the cold water fish. Is this OK or would I need a heater?

Sorry for all the questions, but I would rather not get a fish at all than set it up in an unsuitable home!

60 litres is a bit tight but just about ok for one lone male. They are fine to keep alone.

If you want to introduce females you'll need a significantly larger setup with lots of cover, vegetation, hiding places etc. as males are horribly aggressive when breeding.

They can be kept in temperate water.

Thanks TreeSparrow.