Question about Zebra Danio and crowntail beta?

There are a variety of small heaters that you can get however, if the water is consistently 78 degrees without a heater you do not need to worry, the water temperature is fine. Be wary of adding too many fish to a 3 gallon tank. I'd suggest getting no more then one fish that will not grow to be more then 3'' in length and that is a bit crowded. You could consider getting snails or ghost shrimp for algae eating purposes. I would stray away from plecos since most species grow rather big. Tetras would add a bit of color to your tank and you could put a few of them in there.