0 Kh


Junior Poster
Jan 21, 2009
What is the recommended way to boost Kh in a planted tank with CO2?

I'm a newb with planted tanks and am wondering how transferable techniques that I learned with a low plant tank are to a planted tank with CO2. In my low plant tanks, I just add a palmful of crushed coral in a fine mesh bag to the filter and it keeps the Kh at either 3 or 4.

I'm converted one tank to a planted tank with ADA AS and pressurized CO2 a week ago. I didn't add the crushed coral because I thought the Kh wouldn't drop with the CO2 injection. Tested it last night and it was zero. Ph is 6 (or less as this is the lowest reading on my test.) Plant growth is telling me this is a problem as at first the plants really grew but have since slowed substantially and I'm starting to have some algae on the root.

Thank you in advance for any advice you can give me.


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
Low KH is good for plants, not harmful - with a very few exceptions. CO2 will not directly affect the KH, either up or down. But, lowering the pH with CO2 may cause any carbonate rocks or gravel in the tank to dissolve faster, thus increasing the KH.

Are you adding any fertilizers to that ADA AS tank? I'm pretty sure you need to add potassium, at least, since even Amano recommends that. Potassium has to come from the water column, not the substrate, as I recall from one of Tom's recent posts.


Junior Poster
Jan 21, 2009
Hi Vaughn,

I am surprised to find the Kh at zero. Based on what I've read, I thought the plants were using Kh to make carbon in the absence of CO2. So I thought the CO2 addition would stop the Kh usage. I'll get my DC in later today. Maybe I don't have as much CO2 as I thought. But the plants sure took off in the first three days and now have really slowed down and the crypts are suffering.

I've been dosing potassium, nitrogen, phosophorous and traces. As this is my first time with dry ferts and its a 22 gal tank, I've been dosing

1/8 tsp KNO3
1/32 tsp KH2PO4
1/8 tsp K2SO4
1/32 tsp traces

three times a week.

My understanding is that those are the dosages for a 10 - 20 gal aquarium.

I'll up the dosage some and see how that helps.

Thanks for the suggestion.