Hi gup gup.
Sorry to hear your platy's in trouble.
As we don't know your water quality, here's what I suggest you do...
1) Water change - remove about 30%, and replace with RO water. Do this for a couple days, or a week or more, depending on fish symptons.
2) Vacuuming - get a flexible tube, place one end into the water, suck on the other end, and let the pressure 'pull' the water out into a bucket.
3) Obtain a water testing kit as soon as you can.
4) Keep reading the forum for advice, and if possible post more questions, once you have more details.
By changing the water now, and regularly doing so will maintain the quality of life for your fish/es.
Keep us posted, and good luck.
