formalin is one of those chemicals you need to use in synergy.. meaning its best used in conjunction with wither meth green or acriflavine. Acriflavine is what I'd recommend. (We use that in a commercial environment) in most cases....
Tonic is what most people use at home. can also be used with formaldehyde... but u dont have to. I'm a bit etchy when using formalin... carcinogen and all....
tonic and salt is a good combination.
do daily water changes, and top up the water with a mix of tonic and salt as appropriate. you need to remove any carbon from your filter cause it'll get rid of any medication, and you can keep the temp where it is or even at 28. neons do not like high temps in my experience though.
But you have discus in the tank, so no matter what remedy you choose to select to get rid of white spot - stick with it, and dont combine a whole lot of stuff.. a chemical concoction will not go down too well with them :lol:
HTH! :bounce: