First off, FHs are a combination (hybrid) of 2 Central American species) that originate in hard, mineral rich water.
Using RO water which is devoid of minerals, could be causing enough stress to compromise its normal immunity.
I would use normal tap water, or at least mix the RO with 50% tap.
It sounds like your tank isn't cycled which can be creating even more stress, and immune suppression.
It often takes 6-8 weeks before a tank is cycled enough to add non sacrificial fish.
Ick (white spot disease) often takes up to 2 weeks to cure using a copper based med, or just using salt (3 lbs per 100 gallons)or a quarter of that for your 25 gallon tank.