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8.2 pH in Walstad tank, even though tap water is 7.0...

2686 Views 18 Replies 7 Participants Last post by  MsRed_BR
Two months ago I set up a 20 gallon long Walstad tank, which was intended for my Southeast Asian community (Betta + Harlequin Rasboras).
These fish like soft, slightly acidic water, and I was hoping to eventually make it into a dark water / biotope type tank, using botanicals.
Well, this tank has steady 8.0-8.2 pH, even though my tap water is 7.0 (and usually drops to 6.8 or even 6.6 in my other aquarium, where the fish currently are).
Plants are not doing too great in this tank either. Lots of yellowing, holes, misshapen leaves, stunted growth...
Full spectrum LED lights on for 5 hours, off for 4 hours, on for 5 again, then off all night.
Is high pH a common characteristic of Walstad tanks? Is there anything I can do to lower it to a 7-ish?

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I have two Walstad shrimp bowls by the window and pH goes up from 7.4 to 8.8 in the afternoon when sunlight shines through. I have no fish, just shrimp and snails and I feed sparingly. KH is around 4, and gH around 8. Elevated pH is caused by photosynthetic removal of carbonic acid aka CO2, but I heard that super oxidation of minerals can also raise pH as I can observe oxygen streaming vigorously in afternoon sun.
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