I don't think that leaving the tank fallow is really something that makes it so that you will never get ich again. This is my opinion on it. Healthy fish don't get ich. Relate this to humans because it is easy to do. If you are healthy the odds of you getting sick are low (unless exposed to high amounts of a sickness causing bacteria/virus) Add a stress to your life (like trying to pay for your fish tank, having Deltec tastes on a Seaclone budget) and this will lead to a dip in your immune system which will make you susceptible to sickness. I believe that the same is true for fish. There will always be ich in the water, it can never totally be knocked out. However, a healthy fish, just like a healthy human, can fight off sickness (generally). If you provide stable water quality and predictable living for the clownfish the odds are that it will make it through ich. The problem with brook is that it is very invasive and will kill fish before their immune system has a chance to really fight it off. This is were fw dips or antibiotics come in handy and will save the fish. Is this fish wild caught? FWIW brook has almost been wiped out of the CB clownfish population. Some people don't know how hard it is to take care of wild caught clowns and if we didnt have CB clowns now there is no chance that they would be considered an easy fish. I think we can thank the CB program for making this an easy fish to take care of. I think the FW dip is the best coarse of action with sick fish because they can live in fresh water for quite a while (I had a purple tang with SEVERE ich from shipping live in FW for 3 hours before I put him back in the tank) The FW dip is all dependent on the fish though, while 30 mins is great, if the fish doesnt appear to have a problem with the water, longer is better. I am sorry for the long response but this is my take on sick fish in general.