have all tanks on a single loop to allow for a single point of interaction for filtering -- but also allows disease to spread to every single tank.
here, has every tank on gradually increasing filtration tank-to-tank. He moves fish to more stable tanks the longer and more healthy they've been at the store. You see all the new fish in the new tank and the healthier -- generally not as exotic stuff in the latter tanks. And some of the super exotics in the cleanest tanks... two reef tanks and a curing tank. I think one tank has aggressive fish and ich. The rest are clean.
...every other LFS
has ich in almost every tank
high turn-over rate, constantly incoming new additions -- its really hard to avoid if you think about it.