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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?
TIA.

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Agree with @mistergreen about the soil. The ordinary nitrogen cycle usually brings down the PH. I've become a big fan of pond snails. They will take care of any yellowing leaves and the extra poop might even lower the PH over time.
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That's what I had figured would happen...
I've got almost 1.5" of organic garden soil, then almost 2" of black sand (the inert Petco sand).
Oh. Sorry I didn't notice this before. You likely have some anaerobic spots in your substrate. Poke around with a long thin object and release the H2S gas that's likely underneath. Most of it will be released into the surface air. But, maybe it will lower the PH on the way out?
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