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...