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If nitrates are at 0, and tank has been up for 3 weeks, how was it cycles? Ammonia turns to nitrite, then nitrate, so a cycled aquarium should have nitrate.
Try adding an air stone to your DT till you get a skimmer, your oxygen levels are probably to low and the fish are suffocating? You may just get a lot of salt creep from using one

honestly I've had the same problem with my tank since i started it. Ive never been able to keep more than 1 chromis. (It is a blue/green). my tank has 4-5 tangs, other wrasses and anthias as well as small perching and burrowing fish, i have an extensive invert list and corals and nothing is unhappy or off with the levels or system. But if i add more chromis to the system, my same one always prevails and the others turn up dead 1 by 1 every couple days till I'm left with 1. I have a 200g tank and a 275g system, so it is plenty big, i have just found them to be butt holes and either kill off the others or stress them until they die.
But even if the tank was not cycled they wouldn't have died in three hours, that's what I don't understand?
are you using tapwater or ro/di water?
Something is off in your chemistry, I would hold off on adding fish till you can figure that part out. I have seen fish die in no time at all in an uncycled tank (customer service calls) but more than likely, you got something toxic in your water.
I am completely new to saltwater and am setting up my new (first) tank. I am only chiming in because the guy who is helping me get started was just talking about chromis with me the other day and was saying how some suppliers can keep the fish they catch for days before delivering them to be shipped out. The fish could be completely stressed out from that before you even get them. He was telling me about SSC (short supply chain) fish where they have to go from collection to export in 24 hours. Not saying it isn't your tank as I don't even have water in my first one, but I just thought I would share that the fish you get could be completely stressed out before you or your LFS even get them.
Short Supply Chain - Quality Marine

Sorry, typing too fast... He said 1/4 to 1/3rd die at the LFS when they come in. So if you get one right before it might have croaked, you could have brought him home only to have them expire at home.
Yeah, my wife has had some cichlids for 13 years now. We lost two and so are starting to do saltwater.
That guy told me it is not uncommon for 1/4 to 1/3rd of the chromis to come in at a LFS die.
My LFS has SSC chromis, he just got them in last week and had only lost one as of yesterday. Much better than the 25%+ I might have to try them once I get to that point. My empty tank is sitting there mocking me and begging to be filled, but I am waiting to get my sump and other stuff set up. Maybe I will have water by this weekend!!
i've only been able to keep 3 or more at a time. any less and they seem to just die off.
i also acclimate them a little longer than usual

