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

2690 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've got almost 1.5" of organic garden soil, then almost 2" of black sand (the inert Petco sand).
This is a lot of substrate. Setup is typically 1" dirt and 1" gravel cap. With sand cap, even less. On a larger tank like this you can get away with slightly more than this, but it still seems like a lot. I'm not sure about the pH (maybe anaerobic conditions raise pH?) but I agree with @johnwesley0 that your substrate is likely going anaerobic, causing your plants to grow poorly.

Poke the substrate as johnwesley0 suggested and see if you can scoop some of the sand out (or maybe vacuum it out) to lower the cap. If you notice black slimy roots on your plants that smell bad that is definitely an indication that the substrate is going anaerobic.
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If you're worried about them adjusting could you drip acclimate them over an hour or so?
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