I have had problems where my CO2 was not affecting the pH of my tank even though I had 2 bubbles per second. My pH would never get below 7.4. Someone told me that I had to much filtration on my tank and that was what was causing it (I have a 72 gallon bowfront and I was running a Ehiem 2215 and 2217).
I turned off the 2215 and now my pH is at 6.3 which makes me happy. I would like to get a little more water movement by getting a power head since I had to turn of the one filter. Do I need one rated for 70 plus gallons or would a smaller size be better. I appreciate anyones input.

I don't understand why shutting down one of the filters helped the CO2 issue, unless it was disturbing the water surface too much. A powerhead would do the same unless you carefully aim the flow so it only lightly moves the surface water. The size of the powerhead is not determined by the rated size - that is referring to using the powerhead for it's original purpose, to drive an undergravel filter. Go by the gallons per hour number instead. I use a 120 gph powerhead in my 45 gallon tank and it moves the water very well for me, so I suspect a 200 gph unit would be adequate for circulating your water.

They said that the turbulence in both filters would over airate the water. I had no surface movement at all with both filters running. I actually ran an aerator at night to break the slime on top of the water at night that built up due to a lack of surface movement.
Thanks for the suggestion.

Just for another perspective, I use a Rio Mini 50 in my 75g and consider it plenty.

FWI, how do you have your Eheim return configured - can't you use that as a way to get the right amount of flow w/o adding a power head?

Use one for the surface movement, and the other filter for blasting the CO2 ricj water into the plant beds.

You just need to mix the water , you likely have enough surfce movement.
Be careful not to reduce the water surface movement down, your fish will gasp in the morning.

Regards,
Tom Barr

Are you saying to just gut the 2215 so there is nothing in it and use it as a reactor. All I had in it before was sponges.

No, sorry, I did not say that right.

Use both the Cansiter filters, no power head.

Have one add surface moevment, have one blast the CO2 down into the tank.

Regards,
Tom Barr

I turned on the other filter and it was putting a very light ripple across the surface of the water to help keep the film from building up. My ph went up to 7.3 and I am running 2 bubbles per second (Was around 6.8-6.9 with only the one filter). I pointed the outlet down so it did not disturb the surface and the ph still stayed up. I have turned off the secondary filter and the ph is down to 6.9. If I just run the one filter and a powerhead, is this going to push the ph back up again? This is starting to drive me nuts.

It looks like you aren't using enough CO2, just for starters. Even with surface movement you should get a bigger drop in pH with the CO2. Have you tried figuring out how much CO2 is dissolved in the water?

Another idea is to use one of the in-tank pH measuring devices, converted to a CO2 measuring device. For details, see: DIY Drop Checker - Aquatic Plant Central- aquascaping...a living art

With a pH of 6.9 and KH of 9.0 it looks like my fish should be stressing since CO2 should be 33.991. Yet they all look very content swimming at all levels of the tank. I am measuring the CO2 with a Milwaukee ph 600 meter and KH with an Aquarium Pharmaceuticals test kit. I also checked both with one of the strips and got pretty much the same readings. How does one reduce the KH?