ph reading ~6.2 with new aquaponics setup

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UNC-CH created the topic: ph reading ~6.2 with new aquaponics setup

Weeks ago I started my first simple aquaponics setup. I used an overstocked 20 gallon goldfish tank with a pump flowing water into 2 1.25 sq. ft. Grow-beds filled with approx 25% lava rock (bottom) and 75% (top) expanded clay pellets (hydroton). This drains back into the fish tank (there is no sump.) Two days ago I took measurements and the ammonia and nitrate were at 0 and the nitrate was up to ~40ppm. This nitrate level is good for the vegetables, so I was happy to see it at that fairly high level.

The problem is ... the ph was the lowest I've seen - around 6.2. I just remembered moments ago that the nitrifying bacteria can die at levels at or below that. So, tomorrow I'll be adding a different buffer hoping to raise it.

What I don't understand is how the ph got so low to begin with. I used the same dechlorinator (prime) and the same ph buffer (api proper ph 7.0) that I do with my Oscar and other tanks. They all read around ph 6.8 -7.0. Even this setup read 7.0ph 2 weeks ago. The only change was the addition of the hydroton pellets.

Any ideas? Could the ph drop be from anaerobic bacteria in the crevices of the lava rock and/or the clay pellets. The pellets contain a hollow core which (I assume) slowly becomes saturated with water. I bring this up because I recall that anaerobic units that remove nitrates expel acidic water. BTW- the clay pellets and the lava rock were washed several times before being introduced in the system. I'd say it's the vegetables causing the ph to drop, but the same vegetables were present when I measured the ph at 7 2 weeks ago.

Thanks.
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