Sunfire Platy ...is it sick?

Question
I have had two sunfire platies (1 male, 1 female) along with two Danio fish living in a 2.5 gallon tank for approx. 9 months.  I have a Whisper filter in the tank.  I change the water around 1 per month and when I do, I change out approx 25% of the water each time.  About a week ago, I noticed that the female Platy was puffed out like a balloon, with the scales puffed out like they are standing on end.  She has been moving around some, but not nearly as much as she had been before.  She tends to stay at the bottom of the tank and hasn't been eating.  The other three fish in the tank are fine, with no signs of any issues.  I changed the water a couple days ago thinking this might be the issue, but the puffing has not gone down yet.  I'm just wondering if my fish is "sick" and if there's anything I can do to help it out.  Any help/advice you can send would be greatly appreciated!  Thanks!

Answer
Hi Kim;

She has dropsy, and unfortunately the prognosis is not good. The bloatiness and scales sticking out means that she is retaining fluid because her kidneys have failed. Here is a web page about it and possible causes;

http://badmanstropicalfish.com/articles/article24.html

Your tank is really small for platies. They need a larger tank so they have room to exercise. More water changes are needed too. Once a month isn't enough, even in a big tank. Do a 25% water change at least once a week. The organic wastes that are biologically processed by beneficial bacteria leave an invisible by-product called "Nitrate" that could actually be the cause of the illness. Nitrate is removed by water changes and that's why it is important to do them weekly. It helps keep those organic by-products low and your fish feel better and live longer.

At Your Service;
Chris Robbins