You can gradually increase amount.
do 1/4RO/3/4 tap
2nd week do 1/2 RO
and 1/2 tap.
Test and see where your PH is at.
Are you trying to breed fish ?
Most aquarium fish will adapt to a higher PH. 7.4ph is not high.
What kind of fish and why do you need to lower PH ?
I used Distilled in my small tanks. About 1/2 DI
/1/2tap. But that is for breeding fussy Shrimp. Regular tap worked for my other fish.
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