As i recall you started using RO for water changes so chlorine shouldnt be an issue. Might be worth double dosing water conditioner with water changes to be sure.
Could any detergent/soap/aerosols be getting into the water? Hand sanitizer must be a more common issue in current times.
If someone has suggested it might be the play sand i would want to rule that out. While play sand is considered aquarium safe, there could be an additive in there that is causing this. Syphon out all the sand and go bare bottom for a few weeks and see if that has any benefits. Be careful to monitor your water though as removing substrate will remove some of the beneficial bacteria responsible for your cycle.
And if there is no other obvious cause, then disease cant be ruled out. Are there any signs of disease on the fish apart from the gasping? Red gills? Skin issues? Are they eating OK?