How much is too much PH adjuster in the mix.

Cant seem to find a straight answer to this.. so i got some canna pro bloom PH down. It says 0.5ml per 10L. So i put 1ml in gave it a good mix, let it sit for 5. And repeated. Until i got to 6.3. By now i added 3ml to my 20L. I wanted to go lower than 6.3 as im growing in hydro however didn't want to add to much adjuster. Will that be okay? also for future how much adjuster is toxic for the plants?