pH too low.

My pH has been sitting at 6 or lower (could be lower because that is as low as my test solution goes) for awhile. Plants and fish seem fine but I thought I might try to raise it a little. Did a 1/4 water change - no movement, so must be a fair bit below 6. Thought my test solution might be dodgey, so did a neutral test with tap water, and it was reading 7. So got some hydrated lime today as per Milne's advise and will put in 1/4 cup into the sump and re-test. Will post results this evening after the solution has had time to mix and react.