I assume the reverse osmosis (r/o) unit has the cartridges replaced regularly?
If you use r/o water it shouldn't be a contaminant in the water supply that is affecting the angelfish babies.
Back when I first started in the pet industry we would stir up the gravel in tanks and syphon the water and gunk into buckets, then lug the buckets of water outside to pour down the stormwater drain. Boy that was awful. I remember doing the same thing as a kid. When I first saw a gravel cleaner, oh joy of joys. Then one day I saw another shop using a gravel cleaner made from this plastic drink bottle and clear hose. That was it, no more lugging buckets for me. I couldn't imagine going back to carrying buckets of water through the house.