Repeat after me: pH doesn't matter.
Fish (mostly) couldn't care less what the pH is, provided it is within reasonable parameters and doesn't change all the time. The problems pH cause are through rapid changes, which affect the blood, and that in turn can make them sick.
What matters is the dissolved minerals in the water, i.e., whether it's hard or soft water. Soft water tends to have a low bacterial count, particularly when combined with low pH. Fish that need soft water and die in hard water tend to be stressed more by the bacteria than anything else. This is certainly true for things like Luciocephalus, wild-caught discus, Hemirhamphodon and so on. With at least some of these fish you need virtually mineral free water at pH 5-6, and that makes biological filtration untenable, and zeolite filtration is the preferred method for maintaining such fish reliably.
In any case, if these fish need soft water, I'd not fuss over the pH because acidic water with a high mineral content is of no particular value either way. Concentrate on lowering the hardness. Rainwater is a cheap, safe alternative to RO water and a heck of a lot more environmentally friendly. I use it all the time: all my tanks are 50:50 tap water, rainwater and the fish thrive. You might want to throw some carbon in the filter to remove any airborne pollution, and I do add water conditioner, but given pond fish don't die when it rains my assumption is it's pretty safe au naturel.
Cheers, Neale