tetra medicine tank

I have a 3-5 gallon tank (not sure how big, but in that range) that I may use to medicate fish. It's been set up for a week, with 1/2 old aquarium water and 1/2 new. No filter, but it does have a heater. It would get a 25% water change weekly. Do you think it's wise to put my 9 tetra in this...