My fantail is sick or dying

Question
Hi, I currently have four fish. two blackmoores and two fantails. one blackmoore died last night and we noticed it was dying because it was hanging around in the corner for a few days and couldn't come up to the top of the tank to get food and looked like it couldn't swim properly. now one of my fantails is not swimming properly and it looks maybe a little bloated? and its constantly laying on its side with wide eyes and sucking in air. im very worried i don't know what i have done wrong and how to help it. my dad thinks it wont survive another day. we've only had the fish for about two weeks and Ive done a partial water change once. please help.

Answer
hmmmm, sounds like swim bladder issues...
sorry...how big is your tank?

goldfish produce ALOT of waste and need alot of space and a VERY strong filter...say you have a 40 Litre tank...the filter for fantails and all goldfish really should be made for a 50 Litre tank...there waste makes alot of ammonia and bad bacteria which end up killing them, so this could also be what happened to your fish...if the tank is small...even if you have only had them a short while