Holy smokes
The slime you're describing on the clowns sounds like Brooklynella. Clowns are particularly vulnerable to it, but any fish can catch it, and some never show symptoms but can carry it.
Fungus is pretty rare in saltwater - the salt inhibits it.
Brooklynella can kill within hours - as you witnessed.
That Sailfin... wow, I'm not sure what to make of that. You likely have more than one thing going on there. At first glance, I thought Ich, but it's larger than most Ich I've seen.
My suggestion - quarantine ASAP. Get some Hikari Ich-X or another formalin-based medication. (Do not mix medications). Formalin will address both Ich and Brooklynella and some other parasitic infestations too.
Copper will address Ich, but not Brooklynella (or Oodinium or, or...) so Formalin is the way to go because it will knock out a variety of things. Tangs don't always tolerate copper very well either, and if they have Brook, it won't do any good anyway.
Do it NOW. Treat ALL the fish, even if they don't show symptoms. Leaving an asymptomatic fish in the tank while you treat the others, leaves a host for whatever you have going on, and the treated fish will become reinfested when they are returned to the display.
If you're too late (I hope not)... and you lose all the fish, don't add any more fish for at least 4-6 weeks to interrupt the life cycle of it, and QUARANTINE all new arrivals going forward.
An ounce of prevention is well worth a pound of cure.
As the weather starts to warm up, Brooklynella will become a lot more prevalent. It's that time of year.
Jenn
