I've experienced these situation in great depth before. When I first started, through "professional advised" and I quote " marine ich will be dominant at some point and your fishes will get immune to it." This is so untrue and rubbish!!!! I wasted a good 1 year staring at my DT with ich (on and off whenever a new fish was added). I've the risk of New fishes not able to battle the immunity against ich. Unfortunately your only option is to leave the DT fallow for at least 8 weeks (given you have corals). Finally, Stripped down my entire tank last year due to marine Ich outbreak and diseases. One must dedicate time and passion. What I've experienced will be there is no short cut in this hobby. The importance of having a quarantine tank is Essential. If you are not able to quarantine the new fishes, at the very least perform a fw dip prior to introducing them into the DT. Having multiple layers of quarantine and observations will effectively give your livestock a greater chance of survival. The grave mistake I've made was to rush into the hobby and lost a substantial amount of live stocks at great cost without doing greater research. I'll just treat all fishes going into DT be it disease free or not. Good luck ! [emoji4] Sent from Singapore Reef Club mobile app