There is a BIG difference between just "keeping Discus (tetras, etc.,)" and successful breeding. I would imagine that since you mention keeping Discus and tetras that you are not attempting to breed or raise Discus. (Otherwise you would certainly not place these two together! And smaller tanks work better than a 75g for breeding, anyway.)
First, Discus do much better w/ large water changes. Big eaters create big bio-loads, hence many H2O changes. If you're going to a lot of trouble to bring down the pH in the water you will have much trouble every time you need to do a water change. (Just as fshfanatic mentions.)
Second, many, many farm breed tetras will do just perfectly in moderately alkaline waters with a pH of 7.6.
With higher KH it is very hard to significantly lower your pH using peat, or anything else really. It's not just the pH, but more importantly the KH and GH which buffers the water and resists changes.
Third, you may actually be able to keep some of the more tank-breed Discus in harder water, also. Personally, though, why bother!!???? Unless you really, REALLY, love Discus. Then, spend the money to invest in some proper equipment.
If you just want some larger, graceful cichlids, Angelfish have been tank raised for so many years that they'll do just wonderful in medium hard water.
Trust Me! I tried for over a year to get some Apistogrammas happy in harder water. I sent a few too many to an early . . . .well, killed some. Then caved, and now own a proper RO unit. (Unfortunately, I could not give up the LUST to have some of the softer water Apistogrammas. A. baenschi in particular! )