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

2685 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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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).
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.
Yeah, the tank is filled with bladder snails - dozens of them (as you can probably see in the picture).
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?
Definitely worth giving it a shot!
I'll try it tonight, after I get off work.
Thank you. 👍
I see a tank filled with plants, but no fish. Apparently, there's a healthy excess of photosynthesis driving pH up with all those plants, but no process driving the pH down--nitrification from a filter, CO2 from fish, decomposition in the soil.

I would remove some of the sand. A sand layer almost 2" thick may have altered the tank chemistry, and it is affecting the pH. I can't see how this would affect pH, but it might cause substrate to go severely anaerobic. Also, make sure that water contains enough calcium and magnesium (GH above 5 degrees General Hardness).

Finally, I would start adding fish to the tank. Most fish can adapt to a higher pH. Unless you are breeding softwater fish, the pH shouldn't matter. Fish will provide nutrients and CO2 that will help bring the pH down and correct some of the nutrient deficiencies in plants. With your extensive plant growth, you could put in a LOT of fish.

Plants need fish; fish need plants.

Many people would love to have this kind of plant growth.
Thank you for your feedback, Diana. I feel honored (as I should). 🥰

I'll remove some of that sand this weekend, then.

The plants had been growing in a nursery tank before I added them to this one, so most of them were already fully grown by the time I put them here.

Some adapted quite well, others (like the amazon Swords and the Anacharis) went from being very green and thick, to becoming yellow-ish and thinning considerably - in the Anacharis case, or becoming misshapen - in the Sword's case.

I did the GH test and the GH is only 2 (!), while KH is 12. No wonder my plants were suffering... 🤦‍♀️
For comparison I tested my main tank (not a Walstad) and GH is 11 there.

The tank was cycled on day 6 (I used Tetra SafeStart Plus), but the reason why I didn't add the fish back then was exactly because I wanted to make sure the tank's pH was acceptable and stable. I'm still afraid to add my seven fish in there, and hurt them somehow... 😳
Maybe if I did a huge water change (while vacuuming out some of that sand), then added them right away, the pH would still be pretty much my neutral tap pH, so they'd have time to adjust? 🤞😬

Well... I can predict this will be a busy weekend. 🥴

Thank you all for your help! 🙌
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If you're worried about them adjusting could you drip acclimate them over an hour or so?
I'm actually considering the long term effects, especially on my Betta's fins (he's a rosetail and fin nips a bit), higher alkalinity might damage a fish's slime coat, or at least reduce it, making them more prone to certain health issues, add that to his own genetics, it could be a ticking bomb.
Not only that, but Ammonia becomes much more toxic in higher pHs, which means you gotta be extra vigilant if there's ever a spike.
We'll, those are my main concerns. Not the acclimation as much.
Wonder shells work wonders.
Thanks! :)

Yes, I've revamped the whole tank on Saturday, uprooted all the plants, sucked out 90% of the water, removed lots of the sand and I also added a Wonder Shell to it.
I have also stopped filtration/aeration (I had two small sponge filters running in the tank before), in order to "trap" CO2 in the tank.

Latest water parameters are 7.2 pH / 3 GH / 4 KH.

May tap water has 6.8 pH (right out of tap) and 6.6 pH (rested) / 3GH / 2KH, so I'll definitely need to keep a steady source of Ca/Mg in the tank.

(I do hope my Amazon Sword pulls through somehow. It was my pride and joy... 🥺)

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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.
Yes! You and Diana were right on the money... :)

My tank gets no sunlight, but there was definitely a lack of CO2 in the tank, specially in the evenings (which was when I usually tested my parameters).

I stopped aeration during the day (since my plants are obviously producing enough oxygen) and both of my tanks had a huge pH drop (from 8.2 to 7.2), and that value stays true for the whole day now.
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