There are too many fish in this small a tank as already noted, but in the short-term there are things you can be doing while waiting for the necessary solution which is removing these fish to other tanks or the store.
Reduce feeding. It seems there is a lot of detritus building up, so less food in means less waste out. Feed sparingly, and every other day won't hurt. Do weekly 70% water changes and clean in the gravel thoroughly at each, and keep the filter well rinsed. All of this reduces organics.
See if you can find out the GH (general hardness) and pH of your tap water, check the water authority's website or call them. We don't have this data, but livebearers like platies need moderately hard water and if the water should be on the soft side they will slowly weaken.
