You should typically only medicate for a specific issue. If it is bacterial I use Furan 2. If it is ich, I would either use cupramine or treat by hypo salinity.
I havent added any new fish to my system in over 2 years, so I dont keep one running all the time. If you are still in the stocking phase of your DT, it is best to just keep it running while you are stocking fish.
I keep it up and running before I have fish. Use pure ammonia to keep cycle alive and do a huge 95% water change from dt when I get a fish. As nh3 is topically sky high from over doing the ammonia.