Help With Co2 Dosing

[confusion]

I have a 125G tank that I'm trying to plant. I bought the Milwaukee co2 regulator/ph controller set. I have just over 2 wpg.
Here's my issue: After calibrating my ph probe and sticking it in the tank, I'm getting a reading of about 5.8. I checked my kh, and it comes in at less than 10ppm - my test kit can't give me a better resolution than that.

When I run my parameters through the calculator here: http/www.csd.net/~cgadd/aqua/art_plant_co2chart.htm depending on the kh number I use (between 1 and 9 ppm), I have between 4 and 23 ppm of CO2.

So my question is this: what do I set as my target ph? I'm quite sure I don't have enough CO2, and I'm getting overrun with algae and the plants I have in the tank are not doing well at all. It looks like I would want a 5.1 target. My major concern is that I'm playing at the extreme end of the spectrum for my tester and small changes in ph result in big changes in co2, probably because of my low kh.


Also, is a ph of 5.1 even reasonable for fish?

 

[mgamer20o0]

the change to the ph doesnt effect the fish so no worries there. most people drop their ph 1 point that gives you 30 ppm co2 or so. just bring the co2 up slowly and keep a eye on your fish it should be ok. you might also want to look into a drop checker. its a good way to measure the co2 in your water also.

 

[confusion]

I have to say, I think I'm missing something.

Earlier today - no CO2 dosing started:
ph: 5.8
kh: <10ppm

Immediately after 50% water change (about 4 hours ago):
ph: 7
kh: <10ppm

Now - with CO2 dosing, set point at ph of 6.5
ph: 6.5
kh: <10ppm

Tap water:
ph: 8.4
kh: 30ppm

I have one of the red sea instant co2 indicator deals, and it says that the current co2 level is over ideal. (it just identifies too little, just right and too much). BUT, when I run my current stats through http/www.csd.net/~cgadd/aqua/art_plant_co2chart.htm, I come up with 5.6ppm of CO2, assuming a generous kh of 10ppm, which is clearly isn't.

It seems like my kh is out of whack. If history holds, my ph will continue to drop back to the 5.8 range and my CO2 will not kick in again, as the set point is at 6.5. What on earth is happening??? My tank is getting to be full of algae and the plants are obviously not doing well.

 

[mgamer20o0]

did you let the tap water sit out or use a air stone on it? do you have any thing in your tank that is effecting the ph?