Ok, while it's on the upper tolerable... Slowly raise it to 28-29 in the following week. Warmer water increases the discus's metabolism and can help their immune system fight off pathogens.
If you could relocate the glass, cories, bn for the time being and raise the water temp up to 30-32 would be even more beneficial. some peoples suggest to go even higher. It's not a complete treatment but it put more chances on the side of the fish. Some parasites and other microorganisms that might contribute to HITH don't thrive as much at higher temperatures.