It sounds like you have trace amounts of copper or other heavy metals (mercury, lead, arsenic, etc) in your tap water that's killing off your inverts. Invertebrates are hugely more sensitive to that kind of thing than fish.
I'd suggest getting a copper test kit and testing the tap water. If it's another heavy metal, there probably won't be a test kit for it. You may be able to get an idea from your municipal water department, but it won't cover what is introduced from the pipes between wherever they test (usually at the treatment plant) and your tank.
What you really need to do is start using RO water. With a tank your size, you'll benefit from purchasing your own RODI system rather than paying for water from the fish store. What I can't guarantee is that it will solve your problem, for two reasons...
First, if it is copper or heavy metals, it's going to have been absorbed by your live rock and stubstrate, and simply changing the water won't get rid of it. What you'll need to do in this case is remove it over time by using absorbtion media, such as cuprisorb or Poly Filter. This will take months. Of course there is also...
Second, the possibility that it's not your water at all and that it's something else. However, in my mind, the water is your #1 culprit. The problem you have though is that you have no idea what's in your tank because you don't even start with pure water.
