It depends on your stocking level, if your tank is lighting stocked then the surface area is enough to keep the gas exchange going, if not then you need some surface movement to increase the surface area and support the number of fish you have, it is also worse the wamer you have your tank so going to be an issue over summer. I know quite a few people who have lost some very expensive fish from relying entirely on filters for surface movement.
Regarding getting them to recover, I usually do a water change straight away with cooler water to lower the temperature and also get some good splashing going on, clean out the filter and restart it and then hope for the best.