I had a couple of casualties last year in the heat wave when temps were hitting 31-32 in the tanks
I also had a low ammonia reading all of a sudden in 4 tanks at the same time exactly in the high temp period which I couldn't explain and contributed to a faulty ammonia test but the test showed 9 in the tap water and it stopped reading any ammonia eventually. Maybe it was the lower oxygen levels and the bacteria was not as functional, I don't know but I got away with just two fish that mysteriously died, one was a betta in a tank next to a window so the temps were worse there, couldn't control them.
27-28 is fine for most fish for a period of time but you don't want any higher than that. You should ensure extra oxygenation, even encourage evaporation by opening the tanks. Keep the temperature in the room lower if you can. Turn off the lights if they are a source of heat too. I did cold water changes which were the most helpful, also ice in sealed bags and ice bottles though they only had an effect for an hour or so and only lowered the temp a degree or two max at a time.
As for the plants, they didn't seem to be affected, not mine anyway and I didn't get algae either.