The heat issue occurs for most of us in North America during the summer. Depending where we live, we can expect higher temperatures than 77-78F, and the air temperature obviously affects the tank temperature. Water is slower to heat up and cool down, but it will adjust to the air temperature unless something prevents it. Heating water is easy, we have heaters in the tanks; cooling is not so easy.
My fish room will easily attain over 80F all summer, and during the "hot spells" (which thankfully are usually only once or maybe twice a summer) it will be 90F and over. I now have an air conditioner in the fish room. This works to keep the room at 80F during hot spells, and a bit lower during normal summer (once the warmer weather hits).
I do not adjust or disconnect the aquarium heaters; if they are working properly, they will not come on until the water cools below their set temperature.
For several years prior to getting the small air conditioner for this room, I just ignored it. With the kind of heat in this room (it is southwest facing with no outside shade, so it gets very hot in there) I could do nothing that would have any impact. The air conditioner is the only impact. But, even so, I am not aware of having lost any fish due to the heat. They were less active, and feeding them less is advisable. Depending upon the temp and the fish in the aquaria, adding air stones to keep the water oxygenated is an option. I never have, but some might find this helpful.
I bought a super room air conditioner for less than a hundred dollars at Canadian Tire two years ago, and if I had a second floor room with a window I would certainly get one of these installed. My fish room is on the ground floor, and I use a portable air conditioner which are more expensive. I prefer the installed room air conditioners, and they are much less expensive.