Yes Martin, high temperatures will somehow damage aquatic plants in the medium (more than a few days) to long-term: at least they will stop growing, at most they'll die. Most plants thrive at maximum temperatures of 26-27 deg C, other hardy ones like Anubias can thrive in warmer waters. Fanning the surface will often help you manage water temperatures below 30degC unless you live in a top floor apartment and that could be enough. I had to invest in a chiller to keep my plants growing throughout summer (set at 27degC) but such an investment is NOT always necessary in a planted setup.
Regards Alan
70g Planted Discus tank, *** filtration, pressurised CO2, EI fertilsation, RO/Tap water, PH=6.5, KH=5, GH=5