That depends on your tank… What is your tap water nitrate? What is the tank nitrate? Are your fish young or old? Is the tank heavily or lightly stocked?it sounds very technical and I have to say I don't have a good head for technical. I do a good 30 litre water change every week so as I still don't reall fully understand I need to ask ... Is 30 litres (tank is 100 litres roughly) enough?
I did about 60 litres this week and the fish have been a lot more active since so maybe 30 isn't enough
For the average tank, 15-25% per week seems enough, but if your tank is heavily stocked or has only young fish, then larger water changes could help.
It might have made more sense if I had not made a typo earlier, I have corrected it above. It is relevant since the OP was asking about the difference between tap water and tank water, and I have seen tap water pH change in both directions at the 24 hour point.As much as the above is all correct, i dont see how its relevant anyway. This would only be true if the OP normally tests tap water after 24hrs, but this occasion tested imemdiately.Tap water needs to be stood for 24 hours because of dissolved CO[sub]2[/sub] (which makes the pH appear lower than it really is), although I have seen a few cases where tap water pH actually rose after 24 hours!
Tank water should be tested immediately on taking it, as you want to know the conditions in the tank itself
Since the OP always tests straight from the tap, its good they've done the same, for the sake of continuity (even if the method is flawed). Had the same been left and for arguements sake had dropped over the 24hrs, this could have shown an even bigger drop than 7 to 6, and further worried the OP.