How to increase and decrease ph levels


So I research this fairly extensively as well as listen to everyone’s great advise. I get my ph-down chemical and use it. ph easily drops to around 5.5-6. Within 5 hours the ph is right back up around 8. I’m using the drops to test ph until the package arrives with digital ph tester. It seems that the aerating of the water in my res. naturally raises the ph anyway. Are my ph drops deceiving me or should I simply adjust the water daily and then completely change weekly to avoid a build-up of salts? The Little ladies are growing fast and will soon be needing to uptake nutrients