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Your observations are consistent with mine when I used hard water. It was very frustrating. Suprisingly, you can grow a considerable amount of plants with high KH water, generally anything that won't grow above 5dKH or so. The most difficult factor to control is CO2. It's extrordinarily difficult to get enough CO2 dissolved in the water, particularly in larger tanks. If you're using higher lighting, then forget it. CO2 misting did help quite a bit, but no matter how much CO2 I got into the water, BBA was always somewhere in the tank. I found nutrient dosing not so much of a problem, although higher levels did seem to work better than lower levels. Riding a fine line was almost impossible, whereas my tanks with pure RO are very easy to run lean.
 

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Point of information: <g>

When we talk about water hardness as it affects plants, aren't we talking about dissolved calcium and magnesium, or the GH? That is what's used for plant nutrition, with a very few exceptions.

KH refers to the carbonates in the water. It's main purpose is to buffer the PH,
although a few plants are capable of using it.

I think plants in an aquarium with a GH of 1 and a KH of 10 would be starving for mineral nutrients.

Bill
This is a great point of discussion, probably the point of discussion. :) It's my opinion, and I understand that others may feel differently, but GH is far overrated. To set some sort of basis for my opinion, I tend to run moderately lit tanks because I can grow anything under moderate light. It may take a little longer, but I'm in no hurry and it's beside the point. Possibly under high light conditions GH may play a factor, but I've yet to encounter a situation where Ca or Mg has played a factor in plant health, and that's a lot of situations. I've got 2 lbs. of GH booster collecting dust underneath my 100% RO water tank.

I know that KH on paper tends to serve as a buffer for pH. But why does it influence plant health so greatly? KH doesn't seem to actually do anything (on paper). Why can you not grow Tonina's, Erio's, or have Rotala macrandra flourish in high KH water? Why do plants undergo extreme die-offs when the KH level is substantially altered? I lost an entire field of Pogostemon helferi when I switched to RO from the hard tap water I was using. That, I don't know, but it's something I'd like to understand. All I know is that from my experiments and observations, it works. KH substantially affects aquatic plants. Since moving from high KH water to low KH water, everything I put in my tanks flourish when I meet their nutrient conditions (NPK, traces, iron, CO2, light). BBA is not to be found. In every single high KH tank I've had, BBA was always present. It may have been in tiny amounts and controlled, but it was always there. With my RO tanks, I've yet to see it.

Having said that, I've run my latest tanks with 0dKH and the only source of Ca and Mg being from my trace fert and whatever ionic content is left-over from the RO water since I don't use de-ionization (TDS = 12 on average). They love nitrate to the tune of 6ppm per day. I don't have anything in the tank that leaches Ca or Mg. I was ready for the GH deficiency, but it never showed up to crash the party. I know the lack of KH and or GH on paper makes sense, but practicing it is truly making me re-think things.
 
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