All you really need is a 10 gallon tank i wouldnt chance putting any medication not reef tested in your dt best route now is to setup a qt and go fallow in your dt 76 days if you dont your other fish may fall ill and no telling that clown couldve had multiple diseases that were not present yet. how many fish do you have currently after the clown? Because thatll also determine you qt size ive succesfully housed 4 fish in a 10gl qt for 76days no issues... now you will have to cycle with bio spira or something simillar seachem stabillity etc and an older filter that has cycled using medications could throw off test results so stay ontop of water changes to insure water quallity ammonia nitrite and nitrate controll. due to your tank being possibly contaminated i wouldnt advise using your water to cycle setup some new fresh water keep the lights low and be religious with treatment and water changes best of luck [emoji106]