The only 'cure" for a fish which seems to be coming down with "something" is doing a partial water change. The water should be "seasoned" for a few days and hopefully mostly free of chlorine and active chloramine. It would be beneficial if the changing water was as warm as the tank (or a degree or two warmer so the fish weren't chilled (another cause of clamping and shimmying).

20% to 30% partial water changes every day or every other day can go a long way towards helping the fish and their immune system to heal themselves. Josh, if it has been close to two or three weeks since you last did a partial water change, go with a couple of 20% water changes before changing a greater percentage later on.

If the tank stabilizes with that, try to change 20 to 40% every week. Your fish's problem may just be funky water. But "just funky water" will kill in time. With water changes the platy may bounce back and you will never really know what the problem was. That is ok. :)

In streams the fish are getting water changes all the time. According to goldfish and pond authority  Steven Meyer (sp?) woodland ponds (not attached to streams but still in constant contact with the water table may see 90% of it's water exchanged daily. Lakes don't have quite the refreshment rate, but they have the wind doing what almost no aquarium aerator can do.

So we, as aquarists, can't do too many partial daily water changes - provided we have enough seasoned water at room temperature to do that. That is Mother Nature's solution.

Thank you Mark for your comments. We should have been advising water changes for a generalized illness from the get-go.

When we offered a couple situations such as the hollow belly and the question as to whether the fish had that wasn't answered, the situation was somewhat forgotten. Always, the cure until a fairly definite diagnosis of the illness can be made, is - do more partial water changes!

Good luck with the platy and its tank mates and with the water changes!

All the best!
u.s.

PS: For those of you unfamiliar with The Tropical Fishlopaedia by Bailey and Burgess, it is chocked full of info. The reading can be heavy in places. But diagnostic questions, such as the 44 disease related questions on pages 188-189 (which are explained in the chapter which follows) offers a great checklist to begin helping a fish which seems "out of sorts." If I had to have one fish book, that would not be it. But it is probably the one all-purpose "trouble shooter" I would (and did) buy.

Of course, the risk with that is that you expose ALL the fish to any pathogens at the same time.  Just like the giant display tanks at pet stores..  

Maybe later when things settle down.  Or play it safer and just use continuous siphon overflows to drain excess water, and trickle add new prepared water continuously.

That is great that your plants are thriving and there is no algae on the tank. Some plants are fast growers/ heavy feeders and can be used to absorb nutrients which otherwise the algae would use. Some floating plants can be used to shade areas where algae might grow - again discouraging the lower plants.

What kind of plants do you have? Is your light from a "standard" hood for a 20-gallon tank or another sort? How many hours a day is the light on?

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I owe you an apology on this one:

"Uncle Scott, on my last entry you asked, 'Where do you hale from Josh?' I wasn't really sure what that meant."

I try to disguise the fact that I am among the worst typists and spellers in the world by highlighting a comment in Guppylog, left clicking and copying the passage. I then paste it into MS Word and (looking under tools) click on Spelling and Grammar. I then run a spell check and change my GL comments accordingly.

I was asking, "Where do you hail from?" - as in where do you live (in a general sense like NE Illinois or upstate NY)? Probably it is best for young people to be less specific about where they live, even though the members and visitors to Guppylog are pretty safe people to be with.

The problem with hail and hale of course is that the spell checker can't tell which one should be used in that situation. (This problem even appears in one of the commercial aquarium magazines, which is trying to save on editorial expenses.)

You were asking about live and prepared fish foods. Sometimes there will be a local distributor or an aquarium club near a city or in a section of a state or province that could be recommended.

Not always correct, but I would always try to show respect to you or anyone else by doing as much as possible to correctly type out a comment or question. (Heck the grammar checker just noted the wrong selection of a punctuation point.) That is sometimes called Netiquette or Internet etiquette.

For more on that, see something like
http://www.albion.com/netiquette/

It is to your advantage to always proofread (or have a family member proofread since we always miss some of our own blunders) and spell check a comment. You will almost always get a quicker and more complete response if you have better-looking copy. On some lists and forums a wall of indifference will meet badly typed questions or the writer will even be flamed by the membership. GL members tend to be more gracious, but they are still human. ;)