When did you do the water changes? It's possible the drastic reduction in nitrates has caused problems because everything would've stabilized at that higher level assuming it was a fairly slow increase and not a sudden jump. It's also possible your corals are ticked off from the cloud of debris. That shouldn't kill them but it might take a few days for them to open back up (which is why I asked when you did the water changes). What will also cause a decline in corals (from my experience) is when algae gets on them. I lost two corals to this. If you see algae getting on them then get it off. If your parameters are still in a state of flux that might also tick off some of your corals but if this change in them (closing up) just happened I think it's the sudden drop in nitrates that might be the issue. Now you need to get your parameters stable again.
Something else that was mentioned is flow... Do you have powerheads in there? I don't see any. If there aren't you need to get some of those to increase flow in the tank. I also killed my first two corals early on from having no powerheads. They survived for a little while but when they started to decline it was a rapid demise. I couldn't believe the difference when I added powerheads and bought two new corals that were the same as the ones I lost.
Something else that was mentioned is flow... Do you have powerheads in there? I don't see any. If there aren't you need to get some of those to increase flow in the tank. I also killed my first two corals early on from having no powerheads. They survived for a little while but when they started to decline it was a rapid demise. I couldn't believe the difference when I added powerheads and bought two new corals that were the same as the ones I lost.