do you have an rodi system?
if not then check out
for how to make distilled water at home...
and buy discus minerals on amazon
and start pre-mixing your water before you change it...
ideally you should aim for around 7.4ph
best way to find out is to mark your bucket put 10 lines....one for 10% one for 20%, 30% etc...
then start with something like 80% tap and the other 20% distilled...this should give you the target you're looking for as 8.5*.8 is 7.2 as you're diluting it with 20% distilled
as you have a 20gallon I would go slow with this method probably only changing 2gals at a time...maybe water change every 2 days?
don't just change the water from 8.5ph to 7.2..that alone can kill your fish
change it maybe every couple of days 2 gals (10%) (1.6gal tap + .4gal distilled)
after 10 water changes and achieving the target you're aiming let's go with the 7.2 as it'll probably go up a bit to 7.5-6
then you can go back to doing your regular 20% weekly water changes using the same method...but for 20% (4gals) (3.2gals from tap + .8 distilled)
when/if you do get an rodi which is way cheaper than making distilled at home overtime....
thats when discus minerals come into play
lets say you want to do 20% weekly...
fill a bucket with 4gals and add tea-spoon...wait 24h then test the water in this bucket depending on what you get you'll have a ball park of how much or not to add
once you figure out the magic tea-spoon number you just redo the same for future water changes...4gals + whatever many spoons it took to achieve the target
and because you already know the values...you don't need to retest or wait 24h..you can just change the water