Another thought after listening to your saying the fish were noticeably heavy breathing too - platy's need lots of oxygen. They are fish that get their oxygen through their gills as opposed to Gourami's who have the amazing ability to gulp air from the surface of the water. Gourami's possess a unique labyrinth organ behind the head that allows them to extract oxygen from the air, supplementing oxygen attained through their gills. Oxygen reserves in water can easily get depleted and many factors can contribute to this including overstocking the aquarium, overfeeding your fish, which results in an increased waste production, and not keeping up with regular water changes.
As a minimum you should have the outlet pipe of your pump pointing up at the surface of the water to ensure that it is agitated, which helps create oxygen in the water. You could also get air stones, and even live plants that will help oxygenate the water.
I have found all this by accident. I came back one night and found that one of my fish had knocked the outlet pipe off of the pump, and it was laying on the gravel. This meant that the water had not been agitated probably all day. My Gourami's and corys (who can also gulp air) were constantly shooting up to the surface to get oxygen, but my 2 beautiful adult 8 year old Boesemani Rainbow Fish were both dead on the bottom. They looked absolutely fine, but had died presumably due to oxygen depletion in the water.