what is wrong with my fantail gold fish

Question
Hi, I purchased these two fish on sunday and one is ok, the other one at times seems fine but keeps laying on its back constantly.  Also the water has gone murky already since today (Tues). The well known pet store where I purchased the fish are useless at asking advice.  Should I go and purchase another filter as quite possibly the one I have is not good enough.  More importantly what can be causing the fish to be on its back,  the shop told me the fish was fat and it is! So just not sure what to do next, I just want the know what is wrong.  Thank you

Answer
Hey.

The floating is not good, hes bloated. dont feed him for a week. Feed him one crushed pea a day. Goldfish need 20g each for one fish and 10g each thereafter, so you need a 30g tnak for your 2 fish. (theyre messy). If you are using a 10g, get a small canister filter to clean the water better