Fish very big, what is this?

Hello everyone! I have a platy...? fish that is very, VERY big, she has been like this for quite a long time, and recently she got even bigger. She looks like she's going to explode soon. I have a male in the aquarium, but if this is pregnancy, she should have had a lot of babies a long time ago, which there never was (although the other fish may have eaten the fry if she had any...). Could someone please suggest what this could be, and possibly how I could help her? The fish food I have is Hikari goldfish or koi food (don't remember right now which), does she need a different diet?

I would have to say worms or some other internal parasites, unfortunately livebearers are often riddled with them these days.

You can find dewormer for fish at your LFS, I forget what the essential ingredient is called, @Colin_T would know...

If the fish has been fat like that for more than a couple of months, and it is eating well, then it probably has intestinal worms (as suggested by CaptainBarnicles). See section 3 of the following link for treating fish with intestinal worms.

Hello Dove. A couple things I do. First, and most important is to make sure the tank water is near nitrogen free. Review your water change routine. If you're not removing and replacing half the tank water weekly, you need to work towards it. Also, most fish foods don't have near enough fiber in them. I add a little psyllium soluble fiber to my fishes' diet. It helps with digestion and is very good for their immune system. I put together my own fish foods from a combination of different brands and add a small amount of the fiber. If you're interested in the specifics, let me know.

10 Tanks (Now 11)

Thank you everyone for your replies! (Sorry this is so late) I have been monitoring my aquarium, and found a cute little yellow fry today!I think my fishy is just pregnant, but when the fry are born they quickly get eaten, which is odd as I have a lot of plants in the aquarium. Ill save the info about treating parasites and good fish food, thanks again!!

Well, maybe there are more. Fry can be good hiders.