no a goldfish is the last thing you want to put in that tank. your other option would be white cloud minnows maybe 4 which dont need a heater either or you can get some guppies. make sure you only get 1 gender either all females or all males or else you with have many many babies too soon. i would suggest males, they sometimes fight but are better than females. i had 5 male guppies in a 5 gallon tank with a filter so in your case i would just stick with 3. the only other option would be platys and i would only put 2 in there, only males would be best once again.