Aeration is seriously increasing my pH- what do I do now?

[adriagan]

I feel like I have a solid understanding of pH and dissolved gases, but I'm so perplexed by the issue I'm having. I set up a blackwater Walstad for otocinclus and sparkling gouramis (haven't added fish yet, don't worry) and had everything looking very nice- all parameters were excellent and stable, cycle's going great, pH is right where I want it at 6.6. Then I added a small sponge filter, both for extra filtration and for a little aeration/water flow for the otocinclus, and my pH instantly shoots up to 7.6-7.8 and stays there. I knew aeration would raise the pH a bit, but this much?? I've been slowly adding Fluval peat granules, Brightwell Aquatics humic acid (planted/shrimp), and Seachem acid buffer. No dice, still 7.6-7.8. I feel like there HAS to be something I'm missing here, especially since my tapwater is dead on 7. I'm using API liquid tests, the tank is heavily planted (with both aquatics and lots of riparian growth), and I have a lot of biofilm intentionally growing for food. I'm honestly wondering whether the plants are just sucking up every speck of dissolved CO2 in the water, or if the tapwater I initially used was harder than the tapwater I'm testing now? I'm just baffled.

 

[Aiken Drum]

Have you tried aerating your tap water for 24 hours before testing for pH to see if you get a similar increase?

 

[adriagan]

Have you tried aerating your tap water for 24 hours before testing for pH to see if you get a similar increase?

Not yet, that's a good idea. The tap I tested was my watering/topoff water, so it's been sitting for quite a while but not aerated.

 

[Aiken Drum]

If you don't see much difference you can at least rule out CO2 offgassing as the cause.

 

[Andy Sager]

We had the same situation at an import station I worked at. We lowered the pH to match the incoming water the fish were in and the workers added the air stones to the trays and undid all the lowering. You just need to lower the aeration. If you have a lot of plants, they are supplying oxygen so no need for the extra aeration because this type of tank is not designed for a lot of fish to be in it.
What I would do, since at 6.6 the nitrogen cycle is slowed very much and the ammonia is actually ammonium, is without any extra aeration, check the dissolved oxygen level in the tank water over a 24 hour period. ( They do make dissolved oxygen test kits for hobbyists. ) As long as the reading is not 0, there is oxygen in the water for the fish. The higher the number, the more fish you can put in the tank.