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Bert, I agree with your observations about nutrient levels in hard-water tanks. People who are big proponents of EI are, on average, those who live in hard water areas. My own experience is that EI works much better when the KH is high. In my RO tanks, EI hasn't ever worked real well. I get ok growth, and not too much algae, but the plants don't look as healthy as they do with lean dosing. They also grow more slowly and I see more stunting issues.

At the same time, trying to run a hard water tank lean is asking for trouble. I suspect this is a fairly complex issue. It probably relates to the cellular mechanisms that are responsible for how plants transport various nutrients. As one example, high nitrogen levels in a low KH tank will sometimes produce an apparent Ca defficiency, even when Ca is abundant.

Die-hard EI proponents will agrue against this observation extensively. My observations aren't scientifically founded or proven. They're just what I've seen over the past three or four years that I've been playing around with this.

Avalon,

I think the signifigance of KH lies in the concentration of carbonate in the water. It's well established that certain plants, like Vallisneria, can assimilate carbon directly from carbonate. In fact, those same plants don't do well in soft water systems. Carbonate is arguably a hugely important "nutrient" in its own right and its presence probably influences the uptake of other nutrients as well.

One thing that has driven my own attempts to run my aquariums on the lean side is analysis of waterways where these plants are found in nature. Most river and stream systems have incredibly low levels of nitrates, phosphates, and micronutrients. The sediments in the streambed are often no better. Despite this, the plants thrive. They do this despite fairly low CO2 levels and under full tropical sunlight. How can this work? My own thoughts are that the plants are keenly adept at assimilating nutrients from very dilute concentrations. The one difference that we have in our artificial systems is a limited water column. It is very easy for our nutrient levels to swing wildly back and forth from plentiful to almost absent. In nature the continual waterchange keeps things fairly contstant. The sudden depletion of a given nutrient would be pretty uncommon.

It's probably a true statement that a larger number of aquatic plants will do well in soft water compared to those that will do well in hard water. For me, it's just less frustrating to use RO. I love Rotalas and several other delicate stem plants. Many of these just won't live in a high KH. The list of plants that can't grow in soft water is pretty short.
 
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