is it ICH? new fish dying

Hi. I've lurked but haven't posted in a while.
Can anyone give me some suggestions for this recurring nightmare?
I have a 25G reef, just shrooms, polyps, zoos and a great looking
frogspawn. The corals have been doing very well, almost no casualties.
The fish on the other hand haven't been so lucky.
Most of my fish have at some time or another succumbed to what I
believe to be ich.
The curious thing is that some fish get it and die and others don't.
Example, my two PJ cardinals have no ich on them but my recently
aquired six line wrasse is starting to get some.
I've already lost two clowns and two clown gobies to this horror.
Can this really be ich if the cardinals don't have a trace of it? It
does look like the pictures i've seen. Maybe it's because the
cardinals stay near the top of the tank and all the others like to hang
out close to the rocks and coral?

Any suggestions as to what I can use without damaging my coral? I
don't have a quarantine tank...

Many thanks...