2.2 pH change with CO2

My RO water is at 7 pH 1KH and 1GH. I add 1.5 grams shrimp mineral to get the KH up a little and KH now at 3 and GH at 9, but pH at 7.7. Add CO2 and pH drops to 5.5. Is this a safe change, day and night for neon tetras?

 

Negative.

There is a well established relationship between C02, KH, and pH.

CO2: Striking the balance.

If you are adding C02 to water with KH of 3, and the pH drops down to 5.5, then you have a C02 concentration that is poisonous to animals.

You need to keep the C02 concentration below 30 ppm. That means, for 3 deg KH water, the pH should not be lower than about 6.5.

You are 10 x too much C02 and that much is poisonous.

 

Thanks for the link, I’ll turn down the CO2. It’s only a planted tank but I’m assuming you’re saying that the CO2 can be managed such that the pH shift is small enough to not have to mess with altering ph? That 7.7 seems kinda ‘high’ compared to what I’ve seen most tropical fish like. Thanks!

 

You'll be safe if you aim for a 1 point pH drop (6.7). Or use a drop checker or pH controller.

 

I highly recommend using a CO2 regulator with pH sensor to regulate your CO2.

Technically the pH for neon tetras can go down to 5.0, but an average of 6.5 is more than sufficient.

There is not really a need to add buffer salts though. Actually using CO2 in that hardness range is more likely to fluctuate than with a KH of 1 or 5. With 1 KH the water will have quite a stable pH in the low ranges (around 6), because there is no KH raising pH as soon as CO2 is turned off. With a KH of 5 the pH will stay in the higher ranges and not drop much below the acid capacity, because the CO2 is not able to even out the KH and won't add much acidity.

Your KH is in the inbetween range. There the CO2 can neutralize the buffer capacity and add acidity, but the KH is still high enough to raise the pH as soon as the CO2 is turned off.

I'd decide either to leave the KH low and only add to the GH for minerals (meaning change the additive to a purely GH-raising one) or use a more stable/permanent (and long term cheaper) buffer like limestone and crushed coral, moving into the hardwater zone.

 

Thanks for the link, I’ll turn down the CO2. It’s only a planted tank but I’m assuming you’re saying that the CO2 can be managed such that the pH shift is small enough to not have to mess with altering ph? That 7.7 seems kinda ‘high’ compared to what I’ve seen most tropical fish like. Thanks!

The pH is an indirect way to measure the C02 concentration. For a given KH value (3 in your case), use the table to find the KH=3 row. Now measure pH. The table tells you the C02 concentration.

So you don't directly measure C02, but you measure pH and infer the C02 concentration. Adjust the amount of C02 flow to keep the C02 concentration below the 30 ppm safety limit.