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

Two months ago I set up a 20 gallon long Walstad tank...
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?


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.

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.

Uh Oh! I misinterpreted the situation by the tank picture.

Plants going downhill that fast after transfer from nursery tank is not good. Plant deterioration could be from the low GH, reflecting absence of the all-important calcium. Could you not use water (GH = 11) from the main tank for your water change? If not, consider adding Wonder Shells or calcium chloride to the new tank. Avoid SeaChem Equilibrium, unless it's a one-time dose.

If the plants start dying, all those decomposing roots are going to make the soil layer very anaerobic, especially with that thick sand layer. When you remove some of that sand, poke the substrate with a sharp object.

I would put fish and pH on the backburner until you get the plant problem sorted out and the pH within a more acceptable range (less than 8).

Think about the differences between your non-Walstad tank and this new one.