How low is low?


Guru Class Expert
Oct 7, 2005
Hi folks,

Two weeks after starting EI I am having major problems with Fuzz and Green Spot Algae. Following the advice on this site, I have reset the tank & I am starting fresh with dosing, and at the same time increasing CO2 levels.

I feed the CO2 in the inlet of my Eheim 2217 and I have been slowling increasing the levels to see when the fish start gasping, which has not yet happened.

I have measured my KH with two different kits and they read 4 & 5 respectively. With the gradual increase in the bubble rate my pH now hits a low of 6.3 about 6-7 hours after the CO2 turns on in the morning (a lot of small bubbles are now coming out of the spray bar) & it slowly moves up to 6.6 during the night after the solenoid has been shut off, but the fish show no sign of stress.

My question is, how low can I push down the pH without adversly affecting the fish/plants? If I want to keep increasing the CO2 until I see the fish gasping before I turn it up a notch, I may have to go lower than the current levels, and I am wondering at what point I should increase the KH to stay above a certain pH?

I keep a school of Cardinal Tetras, Otto's and SAE's.

Any advice would be appreciated...


Guru Class Expert
Jun 5, 2005
Dallas, TX
Re: How low is low?

pH is just a reference factor when utilizing co2 to decrease it. fish are going to react to the alkalinity (KH), not the actual pH itself. Which is shown by fish not showing stress by waves of pH changes due to co2 injection. Worst thing you can do is it start compensating the pH drop by altering the KH levels. That will cause undue fish stress.

So, just let your pH drop with no worries (other than if it indicates that your co2 is extremely too high). My pH has been as low as 6.0, and is generally at 6.3. This is with various Rainbows, Cardinals, and scavengers (includes several Oto's and 5 SAE's).

Also, keep in mind, that since you need to keep KH at 3d minimum (safe level), even with 60ppm co2, your pH shouldn't decrease below 6.1-6.2 area.