How long have you had your tank up and running? Any new additions? There are things that go on in a tank that we don't always see. Once of them could be your corals. Did you research your corals to ensure they all "get along"? Corals can and do secrete toxins to harm other intruding corals. These secretions if not removed by the skimmer can potentially be harmfull to other tank mates.
also, your tangs what are you feeding them? Caulerpa and gracilias? Two tangs or more in one tank usually will compete for hiding spaces as well as for these food sources. IF not supplemented this could lead to stress as well as the other fish in the tank. Even though it is a 90 gallon, you have 11 fish corals, crabs, shrimp and anemone. What other fish are in the tank? The largest item for stress for the tangs is the foods and hiding places. Is your tank set up for them to "get away" from wondering anemone?
Bacterial infection can come from anywhere really. And unfortunately yes you cannot treat at the same time, however if your tang isn't suffering from ich, you can isolate him and treat, or you can feed him antibacterial food. That way the antibiotics are not mixed withthe kick ich. Remember your skimmer needs to be off for this.
Also, may I suggest, you seem to have a nice tank with great inhabitants. Get a uv Sterilizer. Look around you can find them at a reasonable price and really it is worth running on your nice tank. For the amount you spend on the kick ich and the time it takes to treat, it is worth the hundred bucks. I run one on my 350 reef and 300 reef but not all the time. I only run it during/after a water change when I know it has cycled the tank two times. Then I turn it off. (It isn't needed then)
Temperature flux's in a tank do not lead to bacterial infection or ich in a tank. The ocean is never the same temperature and can change many degrees in a few moments.
