Black Ich Question

first off, your tank is WAY overstocked, and the formalin probably did more harm than good, freshwater dipps are the best way to go in my opinion. You NEED to get a bigger quarantine, the fish will most likely die off in that size tank regardless if they have ich or not. If the squid is capablr of harboring black ich, stop feeding it, hikari has a really good line of frozen foods that have gon through a 5 stage steriliation procces. Get a bigger tank, and use the 65 as the quarantine, Im surprised all those fish can even fit in a 10g.

have;nt used mela fix, although I was in a situation with a male betta(started out with them, I'm always gonna have freshwater as well as salt) where it was recomended that I use it. With good water quality(more frequent water changes) and more frequent feeding, he was back to normal and you could hardly tell he got in a fight(divider on 5g failed) within a week. I say chemicals are'nt needed unless your tank is improperly maintaned,including melafix.

here is a good article on the lifecycle of ich(even though it's for fresh, it's all for the most part correct info for marine ich) and how to use heat to treat ich, wich I suggest you do, as it wil kill ich at 90F. Keep the tank well aerated with airstones (this is the only time I suggest using them) or lots of powerheads (use atleast 10x turnover per hour) and remove any fish that exhibit signs of exesive stress (they will all be stressed, just remove the ones that look like there really affected by it) to the quarantine, where a freshwater dipp will be sufficeint becuase of how sterile you can keep a quarantine.

PLEASE do not take just my word for it, I'm not completly sure if heat works for marine ich or the differences between normal ich and black ich. At the moment my usual site for disease treatment is down and I can't find a site with info as good as it has/had.

Best way to cure Ich:

Remove ALL fish to a quarantine and treat it with copper, then let the aquarium run fishless for about 3-4 weeks. This will solve your Ich problem.

I had a case of ich too. Not black ich though. You need to take the carbon filter out and treat the tank as well as the fish.