Just to clarify, are you asking if the 2 gallon tank can be used as a hospital tank to treat the ailing platy? The answer is that it can be--but you will have to do large, daily water changes to keep the wastes low, and monitor closely to make sure the light isn't spiking the temp too high, nor letting it get too low when it's off. Certainly not ideal, but potentially better than the main tank if the sick fish is getting picked on or if the medication you've chosen will be lethal to biofilter.