Experiment with a bubbling airstone. If you notice the rapid breathing going away after an hour of the airstone then you know it's simply not enough dissolved oxygen in your water. In almost all circumstances, dissolved oxygen issues effects the largest fish in your tank first because the larger the fish the heavier their air requirements are. If you notice that they are still rapid breathing after the airstone is used then you might have another issue. Do you notice any scratching of the gills? Are the eye's clear? Any paleness in skin tone? fins deteriorating? However, an airstone in a planted tank should only be temporary because plants need C02 to thrive, an airstone would drive out most of the C02 needed by the plants to flourish.