Originally Posted by DKRST
Could be a significant shift in pH? The tank went from 7.6 (high ammonia) last week to it's current 7.0.
A shift in pH can cause plant changes (usually temporary), that's true; but 7.6 to 7.0 is not major--although crypts would likely melt at this. My tank pH fluctuates by about .3 or .4 every water change, but that doesn't seem to deter the duckweed.
More likely something affecting the water chemistry, and I think it just occurred to me.
Plants need ammonia, or rather ammonium, as their preferred source of nitrogen. Duckweed is a fast growing plant, and thus assimilates a lot more nutrients, ammonium included. The Ammonia Sorb may have removed so much ammonia that the duckweed was literally starved of nitrogen. I'm going from memory here, but I believe green plants turning white [as you mentioned] indicates nitrogen deficiency.
I tried to find info on Ammonia Sorb online, and can't. That in itself would cause me to never use this product. All I can find out is that it supposedly "removes" ammonia. Some products, like Prime, detoxify ammonia by changing it to ammonium, so the plants would actually benefit. But if the ammonia is actually being rempoved from the water, and in sufficient quantities, plant trouble could ensue.
Echinodorus (swords) take longer than faster-growing plants to show such effects, but within a week or two you may see issues with the swords.
I would certainly remove this product, and if ammonia continues to be a problem, use something that detoxifies it into ammonium. Though if i read correctly earlier, the ammonia issue is now gone.