Well, your best bet is to either wait until your 50g cycles and use your 30 as qt and qt ALL of the fish, sick or not. Only problem with that is depending on where you are at with cycling it may be too late for the tang. So, other option is either buy another tank, which isn't what you wanted to hear. Or what Quite a few people are doing now is getting a rubbermaid storage unit and use the water from existing tank and put the fish in that. Only fish though, everything not fish stays in your 30g. From there you will treat the fish with either copper or hypo salinity. And leave the 50g cycling for another 2 months. I say 2 months because most treatments require a 6-8 week treatment. After that you can put them into the 50g with the trust that it is completely ich free. Now, two things also to consider is that your inverts though the ich parasite can't live off of them, it can hitch a ride. So, anything, regardless of what it is, also needs to be quarantined for at least a month, sometimes longer. They don't need to be treated, just separate. And obviously any new fish you get goes through the same process. You may not have to treat them, just observe them for that time and if they show no signs than they can go in, but if they have ich, the 6-8 week cont down starts after they have no spots on them.