Mine is drilled as well and its a 75 I have 2 koralia 3s You should look into getting two or maybe even 4s would be needed for the amount of flow that you need. Look more into water flow there was a post on here about the movement that it gives you in your tank and how much and what not. I believe this might be your issue. Just having an external pump to pump the water back into the tank is not enough flow. You need some powerheads to circulate water within the main tank more.
Our test kits can test for certain things like ammonia, nitrates and etc. There are certain tests that we dont have so that`s why I suggest carbon to remove it. As far as the chemical warfare Leathers when encroached upon will wage a chemical warfare to back the other corals off.