Skunky wrote:

Forgot to mention the tank is 260 litres.

Skunky,

This tank is far too large for a Hagen CO2 system to help very much. The Hagen system
is rated for tanks of up to 20gallons, approximately 75 liters.

This means that you probably can only drop the pH by one or two tenths of a point at
best until you upgrade to pressurized CO2, or multiple DIY bottles in tandem. Some
people use two, three or even four 2-liter bottles in parallel for larger tanks, but
this eats up a lot of sugar, yeast and time.


The problem is the water in my R/O vat, just can't seem
to get the pH at 6.8-7.0 with a KH of 3 or 4.

Go back to Chuck's chart. At a KH of 3, CO2 diffusion from the atmosphere at
approximately 3ppm will give you a pH of about 7.5. With the Hagen system at full
output, you might be able to drop that to 7.2 for a couple of days.

At a KH of 4, atmospheric CO2 at 3ppm gives you a pH of 7.6, and the Hagen system at
full bore might drop it to 7.3.

Keep in mind that if you have plants and low surface agitation, the plants will
consume the CO2, dropping it down to 1 or 2ppm, and raising the pH accordingly.

So what problem are you trying to solve exactly? Do you want a lower pH? Then add more
CO2. At KH3 and CO2 of 23 ppm, your pH will be 6.6. This will require pressurized CO2
of course.

Also a plant tank can be quite happy at a KH of 2. Here's my tank parameters:

72 Gallons
KH2, GH3
Pressurized CO2
pH 6.4
220 W compact flourescent lighting.

I change 20-25 gallons per week (about 30%). 100% RO reconstituted with Kent RO Right
and Baking Soda. And Leighmo was right, Kent pH Stable is pure baking soda. I use
baking soda from the grocery store.