I did not use RO water, but assumed prime would take care of it. Seems odd they would lived 14 days or so if it was an RO issue? Yes, it was half filled, so 5 gal or so of water.
Could have been bad food choice, too much perhaps leading to bad water quality.
They all died off a few days later. I'm thinking it was water quality as it wasn't a cycled tank. I read someone else suggest full water change every 7 days.
I saw a video where somebody was using a two liter water container, top cut off soda bottle perhaps, but put down in the tank water to...
melafix and primafix are both pretty weak treatment options. I've read that 'actual' fungus infection are very rare and it's almost always bacterial in nature.
However, Kanaplex does treat both and is broad spectrum treatment.
Fuzzy white growth could be calumnaris, which kanaplex would...
I think it gets expensive to target and maintain a specific pH. A consistent way is to use reverse osmosis water, then add seachem Alkaline buffer, acid buffer and maybe seachem equilibrium to add back some good minerals. I think this is how many discus keepers maintain their tanks.
I was trying to help explain why it's important to monitor, especially ammonia and do water changes. And why the fish store guy wasn't giving good advice. If you think it's too much info and confusing, I can delete it.
A little more info, higher pH and temp make ammonia more toxic. Some fish give off more waste, I.e. more ammonia. Lots of variables. Ammonia of 2ppm in 72° with pH of 6.8 would be fine, 2ppm at 79° and pH of 8.0 would likely kill any fish.