Laurence -
I am a little unsure as to why you are certain that the temperature is the culprit for your coral problems. While 84.2 is a little high, it is not really in the coral killing zone. If you have not yet, you should REALLY check out the section on temperature in THIS article:
Reef Aquarium Water Parameters by Randy Holmes-Farley
In that article, Randy recommends keeping of reef aquariums at 76-83° F. HOWEVER, he goes on to comment:
In most instances, trying to match the natural environment in a reef aquarium is a worthy goal. Temperature may, however, be a parameter that requires accounting for the practical considerations of a small closed system. Looking to the ocean as a guide for setting temperatures in reef aquaria may present complications, because corals grow in such a wide range of temperatures. Nevertheless, Ron Shimek has shown in a previous article that the greatest variety of corals are found in water whose average temperature is about 83-86° F.
In short, I think you might be looking at the wrong culprit for your problem. Rather, perhaps there is something else going on with the system.
For one thing, this might be time to take a little more seriously the advice by folks to slow down with the introduction of new corals, etc.
And lastly, how much flow do you recommend you have going on in the system. With 800 gph for the return and 6000 gph for the vortechs, that is only 6800 gph total. And with a 340 gph tank, that's a rate of 20x turnover per hour. I might suggest the addition of a third vortech.
One more thing - have you done much water testing recently? I apologize if I missed that, but this might be a good time.
Good luck with things - I hope things turn around for you. It can be very sad to see a precious coral go!
