To biker:
In western country, ie, US, Europe, hight light means about 3W/US gallon. 1 US gallon ~= 3.7 litre. So if 1 W/litre, it is considered very high light.
As u said, the hardness of water plays a factor. In Europe, the water is generally very hard, ie, from underground water. I rem when i was in France, it is hard to get bubbles from soap due to the hard water.
So what u said might be true.
Btw, I have several A. Ulvaceus, A. crispus, A. Rigidifolius, i will see how they behave under high light(~1W/l) and moderately high co2(~15ppm) and fairly soft water (~4kH)
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