Pleeeeeeeeeeeeease don't take this the wrong way as this is all meant to help...
Let me make sure I have everything right first. You have a 90 gallon tank right? Hang on the back filter? The tank is less than 3 months old? I noticed you first started posting and said that you had live rock on it's way via the mail. That means that after the rock arrived, with die off, your tank would then cycle. I also read that you are trying to scrub off coralline algae and didn't know what amphipods were. All that and I saw that you had a vlamingii tang die and I saw in a picture that you have a sailfin tang. By the way, how did an urchin kill a vlamingii tang in a qt anyway? An urchin shouldn't be in a qt tank if it was there to heal up...from the damage it received while you were aquascaping?
So here's the part where I don't want you to shoot me. My advice is to slow WAY down. You have fish that are not appropriate for your care level, OR your tank. There's many things to learn in this hobby and now that I'm back in it, I'm learning new stuff every day here on RC just by reading. Instead of spending your money on livestock, my advice is to spend it on better equipment, IE: a sump!, and just take it easy. Test out your parameters regularly and let your tank settle in. It's considered new up until a year from setup!
Maybe more can chime in, but I saw a lot of not so good things when I flipped through here.
