2 days later, ph is very low - how do I adjust it ?

Since I have read in many places that for aquaponics, the pH should be somewhere around 6.5 to 7, I decided to mesure the pH of the water in my tower since the beginning. I used little pH strips with 3 colored indicators to determine and I was surprised when I saw a pH of 5 ( or below, since the strips do not go below 5). I was surprise as my vermicompost pH usually is around 7 and 7.5, and that I assumed that my "tea" would be close to that. Also, before starting I downloaded the NYC water quality report, in which they indicate a ph close to neutral ( ie 7ish).

I therefore dived a fistful of vermicompost and stuck a strip in it, to come back to it later (after 10 min or so, so the moisture has time to react with the strip). The color indicator suggested a pH close to 7.5 ( slightly above). So I then measure the pH of the tap water ... And it was a quite surprising 5 !

Anyway. Once I realized that, I looked in my kitchen cabinet and pulled baking soda and baking powder ( both contain sodium carbonate, which should help raising the ph). Baking powder also contains phosphate. I mixed 1/8th of a teaspoon and added a "knife tip" to the sump tank, kept the pump going for an hour, and checked the pH again, it still indicated 5. I added more, and more and... I almost used all the mix. The pH was still 5,  I stopped kept the pump running overnight and measured again in the morning. The pH was still 5 ( and a bicarbonate solution was 9 (or above) so the strips do work). I stopped adding the baking powder/soda mix as I became worried that I would add to much sodium. I therefore turned t the only free source of carbonate I could think of : eggshells ( calcium bicarbonate), I added a teaspoon of it and let it do its thing for the day. In the evening, the pH was raised to 5.5. This morning, the pH was back down to 5 ... I therefore stole a nylon sock form moja sarenka, and stuffed crushed eggshell in it, then threw it in the sumptank. Later today, the pH raised to 6 ! In order to not shock the plants more than they need I decided to remove the sock around 6pm. I will check the pH again once I am back home.

Why do I want to raise the pH ? Well, reading a few forums and othe guides for aquaponics, it seems that the pH is a quite important factor for maintaining favorable condition for the bacteria responsible for transforming ammonium to nitrite and then nitrite to nitrate, which is the form of nitrogen that the plants can use for this needs. Actually, the higher the better - to a point, but it would then become an unfavorable pH for the future fishies to come. This is why I feel the need to adjust the pH up.

Back at home :

The sock filled with crushed eggshell: 

The new pH reading : 6.5 ! it kept rising without the sock in it - probably some of the finer pieces fell though, and kept being disolved. Online sources says that the pH stabilizes around 7 to 7.5 or 8 with crushed eggshels (and other sources of calcium carbonate).