Which fish need heaters?

Yes you should because Both types of fish you mentioned are tropical and need to be kept around 72 degrees. Only cold water fish like goldfish and koi dont need a heater. Oh dont forget the thermometer.

Swordtails can survive 70 and glofish (danios...) can manage just fine with no heater. depends on your room temp.

Best of Luck!

(ps, you will probably get flamed because you bought a glofish. the fishes ancestors underwent biological manipulation to cause the bright colors. The ancestors underwent this, not these fish, though I do object to buying "painted" fish due to being injected with dyes...
But I don't see the fuss on glofish, they complain because a colored fish is in the aquarium, they should read the labels on the food they eat... engineered wheat, rice, fruits, eggs, meats (including cloned animals occasionally)...

Tropical Fish for the most part. I wouldn't say tropical fish REQUIRE heaters, but heaters do keep aquarium water at a set temperature. If you are able to constantly keep your room temperature at a solid 78 degrees then no, tropical fish don't NEED heaters.

Cold Ones!

Cold Ones!

it was a bad idea to buy fish the same day you got the tank read up on cycling a tank weather or not you need a heater is the least of your worries

it was a bad idea to buy fish the same day you got the tank read up on cycling a tank weather or not you need a heater is the least of your worries