bloated bellies

Question
a few weeks ago maybe amonth i went to petsmart and ai bought three platies. one sunburst and two mickey mouse. when i first purchased them everyone seemeed to be about the same size. the sunburst was slightly larger, so i went away to st. croix for a week. my roommate fish sat for me. when i returned i immediately noticed one of my mickey mouse platies is swollen,  im talking ready to burst swollen. it is huge, the belly is round and bulging toward the front of the fish, i can see it popping out from the sides as well, i dont believe that my fish is pregnant because the associates at petsmart assured me that all the fish in that tank were males. i am afraid it may be stricken with disease, unless the unlikely has happened and they are homosexuals wo produce babies. any advice that you can give me would be greatl appreciated. if it helps the fish has bright eyes and look quite clean  and healthy. thanks in advance.
diana

Answer
Hi Diana
Hmmm, I think looking for the bright eyes is the rule when you're buying fish from the grocery store to eat :)  And I can honestly say, I've never heard of a male platy giving birth, though who knows, now a days anything is possible I guess lol....

Ok, first lets make sure they are indeed males.  I know Petsmart only sells one gender of an animal, but that's usually restricted to their hamsters and such.  And, I've had numerous experiences with the fish people there not having a clue to distinguish between a male/female platy/livebearer.  There are a few good chain pet stores out there with very informative people working in them-a few, but....I would take their advice and assurances with a grain of salt.  
Here's some links with some photos:

http://www.liveaquaria.com/product/prod_display.cfm?c=830+1103+1069&pcatid=1069

http://www.liveaquaria.com/product/prod_display.cfm?c=830+1103+2727&pcatid=2727

Those first 2 photos are male platies.  See on their belly, the tube shaped "thing" closest to the tail?  That's called a gonopodium, and that's what makes a male livebearer a male.

http://www.liveaquaria.com/product/prod_display.cfm?c=830+1103+1067&pcatid=1067

That one is a female.  Notice in the same area, instead of the tube shaped gonopodium, she has a triangle shaped fin instead.  Now, watch your fish for a few minutes and look for one or the other.  It's best to get a glimpse when they're sitting still, because the females will sometimes fold that fin up and it looks like a male when she's swimming around.  And also, when they're young, they all tend to look like a female for the first few months.

Now, if the fat one is a female, it's most likely that she's pregnant.  They usually come that way from the store.....almost a given.  Gestation period is about 28 days, give or take a few days either way.  Oh, and females are also able to have fry(babies) every month for around 6 months, with no males around, just from one encounter with a male, because they can store the sperm for that long.

Now, if it is a male(or even still a female, though if it's a female I still say she's most likely pregnant), it's not pregnant, and is most likely suffering from either constipation or dropsy. For constipation, and this is fine for all the fish, feed the fish the inside of a pea split open for a 3 days or so, once a day.  I usually just drop a pea or two in the tanks, and scoop it back out after awhile.  If the "bloating" goes away, you know this was most likely the cause.  If not, it very well could be dropsy.  Dropsy is a condition a fish can get, can be caused from numerous things from poor water quality, internal parasites, cancer, to internal bacterial infection.  Basically the fish's kidneys shut down, and they start retaining fluids and swelling up.  Eventually, the fish will take on the classic "pinecone" appearance, where the scales stand out like a pinecone.  But, it's been my experience that happens towards the end, and the fish usually dies within a few days of the pineconing.  Dropsy is almost always fatal, a few people have claimed to cure it, but without really knowing the cause of it, it's hard to treat.  I currently have a platy that has chronic dropsy, and it's been about a year now.  She just looks fat-like she's pregnant and bloated, but she eats and swims fine, and I don't have the heart to put her down while she's acting fine.  

I would recommend just keeping an eye on the fish, like you said, it looks healthy and is acting fine at this point.  Also keep an eye at the bottom in the gravel area for little fry, they're tiny.  Be sure you're feeding a varied diet, include a pea in their diet once a week or so.  It's good for all fish, I have a big selection of different fish I do that too, just have one I have to "sneak" him the pea otherwise he won't eat it.  
Also make sure your water quality is good, ammonia and nitrites at 0 ppm, and nitrates under 20 ppm.  If you don't have one, pick up a good test kit that tests all those levels.  I recommend API Freshwater Master Test Kit, it's a liquid dropper kit, costs about 30. at the store, about 15. online at drs. foster and smith.  Has all the kits you need.  And, the liquid dropper kits are better and more accurate then the dipstick ones(like the pet stores use).

Hope that helps and hope you enjoyed your vacation, assuming that's what it was :)  and good luck!

Christy