The most important thing of all is they need clean water. Doesn't matter how tiny of a tank you keep them in, clean water is the most important thing!
It is like their air. You wouldn't want to live in a bathroom with a toilet that didn't flush, don't make them live in their own waste.
The second important thing is being warm. These are tropical fish. In their natural habitat 78-80 is normal, higher is not unexpected. Being warm helps them digest and helps their immune system function properly. Now, it is hard to keep the temps stable in a tank that is less than 1 gallon, and most heaters won't fit anyways. Plus you will have to do pretty frequent water changes on something smaller than 1 gallon, you have to do pretty frequent water changes on a one gallon!
Good for you wanting to learn how to better take care of your pets! I have a picture below of my divided tank for my two boys. Obviously this is above and beyond what is needed for happy fish, but I find my tank very relaxing to watch, and it works for me. You just have to find what works for you! If you post more details about your set up, or you keep reading information on this website I am sure you will learn so much about your fish you never even imagined! I know I did. :)