Sick red wagtail platy?

Question
I just bought a red wagtail platy 2 days ago. I noticed today that its belly looks like its about to explode. Is it sick or could it be pregnant? Not sure what the sex is and have no clue how to check.

Answer
hi Stacy: The male platy has an extension on his anal fin whereas the female has an anal fin that is mostly triangular.... so look at him/her closely and see if there is a little bit of extra fin trailing behind or if the fin is more of a triangle.... if your fish is a female... she is likely pregnant... though she could be sick. The stress of going from the aquarium to home can sometimes trigger illness and in females early birth syndrome.  The fact that it has puffed up rapidly could mean illness... so raise the temperature of your tank slow to about 78 degrees and ad some aquarium salt per the instructions on its label.... please keep me posted about the progress of your fish.... dave

ps: if you think she is going to have babies now is the time to prepare for that... a clump of java moss on the bottom will help to protect the new fry... if you can move her to a tank by herself then do so.... also  you will need something to feed the fry...microworms, baby brine shrimp and boiled egg yolk work well.  If you have a filter you will need to either change it out to a sponge filter or turn it off for a little while... dave