A comet or common goldfish need at least 75 gallons minimum, preferably a pond. Fantails I believe are 20 gallons for the 1st fish and 10 gallons for any other fantail goldfish you add in. They are not recommended for Bettas as goldfish are better off in cold water. They can be in hot water but you need more filtration, aeration, and even more food. No idea on the Swordtail but Platies need to be in groups and are a bit nippy.