What is the PH of your tap water? If it is also 7.4 or there abouts then you know the C02 that you were adding lowered it to 7.0.
Why do you want it to be 7.0? You need to look at your C02 regulator and see why it is not working, check to make sure your tank is not empty, etc.
7.0 is neutral which is not a problem for plants. The problem is C02, Light, Ferts, etc.
Also, it is better to deal with your current water conditions rather than chase your water trying to change it. Your fish would suffer if you had to continue adding chemicals to change it.
10 gallon hospital tank.
29 & 50 gallon - High Tech C02 enriched.
50 gallon Pond - Natural.