HELP! Is my budgie sick?

Hello, I have 5 budgies I just added to more! One of them died I don't know why but I do know I knew he needed to go to the vets. Don't take my age to blame because I probably know more about any common animal you can think of than 10 NORMAL adults A.K.A not breeders! By the way im 13 so anyway... The other bird the one that I bought at the time as the one that died is in his own cage now he was with the other bird and is in the same air space as all of the budgies... He looks fat unless I reach in to get him then he will put them down. He still eats and drinks and seems to move around his cage to the best he can but I also only have one perch in there at the time because it is not his final home and I was disinfecting the toys and perches to make sure nothing really spread that way but it probably wont help since they had been in the same cage together. They all do this "fat" thing but I tend to worry about lem-lem A.K.A the new alive bird the most since he was with the one that died i've taken him to the vet as have the others 3 times before... They said he seems fine but he also had his feathers down at the time so they didn't see what he does that makes him look fat. I worry a lot about the birds because I know how delicate they are and how fast they can go down hill. Also that they hide it so well for so long :( They all eat zupreem fruit blend pellets and nutriphase seed for parakeets and also a cornbread stuff the vets had me make so I didn't have to feed fresh fruits its rather neat they love it and it never goes bad :) Anyway They all seem pretty active and loud maybe im crazy and need to cool it plus my mom doesn't let the house get to warm because of the cost what temp should it be I try for at least 70 but it always goes down somehow??? Just kidding I know my mom turns it down!!! :( Any HELP is very much appreciated. Thanks -Nicholas