yep it is unfortuanately, i had a sailfin die on me in 2 days and golden nugget more recently die in 2 days.
luckily for me i work at the shop where i got the nugget so i havew it on credit.
there are ways to pick out a good un but it's hard to get it spot on!!
the main factor will be there belly, you should try and pick one that looks nice and plump is the belly is tought you should try to steer clear. Also go for one with the best colouring as that may help.
although the guy that picked out my sailfin got a nicely patterned one which is the one that died!! luckily he replced that one.