I am planning a large planted tank. I am considering the Aqua Soil Amazonia for the substrate. The informattion says it lowers the PH. Not a bad thing, but so does CO2 injection. If the PH is lowered by the substrate how can I inject enough CO2 for the plants w/out lowering the PH to dangerous levels for the fish?

There are at least 4 ways to resolve this.

I've seen quite a few comments to this effect about ADA AS, but in the distant past, peat and driftwood etc had the same issues. Some make a rather big deal about it.

One, do a massive water change, say 90% and the new water will only be influenced by 10% of the remaining water. Repeat the next day.
Now most of the water is pretty much normalized.
The substrate takes some time to extert the pH depression.

So you can measure things quickly there after.

Two: take a sample of the TANK water, allow to sit for 24-48 hours, measure the pH. do the same thing with the tap/RO etc, whatever you refill with, let sit for 24-48 hours.

Now measure their pH's and KH's. What the differences? You basically correlate a difference between the actual CO2 vs the tank water's CO2.
Add those differences to the values you measure in the tank.

Say the tap is pH 7.5
And the Tank is pH 7.0

When you target the pH, say it's 6.2 for the good CO2 ppm range, you now add enough cO2 to drop it to 5.7.

Also, measure KH as well.

Realize also, that the effect is variable through time.
So the first few hours, the tank/tap after a large water change are similar, two weeks later? A lot different.

3. pH/KH ref drop checker, check the articles and threads here for more on that.

4. pH/KH ref pH probe membrane method, again, check here.

Regards,
Tom Barr

Dangerous pH levels for the fish are lower than I suspect you think. As long as you are lowering the pH with CO2, and not by adding muriatic acid or other acids, the fish aren't going to be bothered by some pretty low pH's. But, adding too much CO2 will bother the fish by suffocating them. pH just isn't as critical a parameter for them as most of us used to think it was, but total dissolved solids is probably more critical than once thought. That's why changing 80-90% of the tank water with tap water doesn't normally bother the fish at all, even though the pH will likely change quite a bit while doing that, but adding a big measure of bicarbonate of soda to raise the KH will bother the fish. At least, that is how I am interpreting what I have been reading here and on other forums.

I have a KH of 1 out of the tap, and I am in the process of switching from eco to ADA AS as well. I know my KH will drop to zero as a result, and my present pH of 5.9 with CO2 injection will probably have to drop to 5.3....does that bother me? no...will it bother my fish? no. The only thing I keep an eye on with this type of drop is I make sure that I have enough agitation in my water column to provide fish with O2. That is it. Co2 doesn't hurt fish as much as lack of O2 does. As it is now, my pH fluctuates overnight because I turn off my CO2 when the lights go out. My fish have never shown any sign of sensitivity to this delta...I even have the same original 10 otos I started this tank with, and they are considered sensitive, I hear.