I think part of the issue here is that when doing a publically viewed tank we tend to put livestock in before the tank system and cycle process are fully vetted. It would have been better to drape the tank and go through the cycle process and system checks in a more systematic manner, but hind-sight is 20/20.
With the snail die-off you will probably have elevated nitrate and phophate levels and it will take some time to get that stabilized. I would skim wet, do water changes, and adjust your temperature control system while leaving the tank inhabitants as they are. You will still probably experience an algal issue especially with the large die-off. Perhaps cutting back on lighting for a while will help though.
Relying on the building AC may be a mistake in general. First of all, you don't know what the air quality is and what its oxygen content is. Second, the air must go somewhere. In other words, if you are replacing warm tank room air with cooler forced air, the warm air needs some place to escape, otherwise you will not actually cool the tank room. With the door and the hood closed, where can the warm air go? The forced air will find a path of lesser resistance, which will be an adjacent room. For cool air to replace the tank room air, either a vent to the outside needs to be installed, or possibly a louvered vent in the TOP of the tank room door. Hopefully in that case there would be a return in the waiting room that would draw the warm air out.
Because this install is internal to a commercial building, you may have no other choice than to install a chiller if you cannot figure out how to use the AC vent properly. And when the building needs heat, won't the tank room get heated too?