I read that Water Hyacinth and Giant Duckweed are the only two plants that take out ammonia AND nitrates AT THE SAME TIME. All other start with the ammonia and only after it's used up will they do the nitrates.
Hmm. The way that's emphasized, it sounds as if you consider it a good thing, but if your info was correct then I'd actually consider that bad. Because if something is wrong and ammonia is accumulating, especially enough that it registers on a test, then you would want
plants to start with the ammonia!
But under normal conditions, ammonia tests zero. It's continuously being produced and is available in some limited amount, just being consumed at the same rate, so it doesn't accumulate. All plants use a combination of ammonia and nitrate under these circumstances, not just Hyacinth and The Giant Duck. If any plant were to totally spurn nitrate because some insufficient pittance of ammonia was around, it would be in serious trouble.
Has anyone noticed a decline in algae since adding duckweed to there tank?
It will help if the algae is because of too much light, because it blocks light. It will also help if you don't have enough plant growth in your tank to accomplish whatever mystical thing plants do to suppress algae, since duckweed grows fast. Of course, you could instead just reduce light, or add other types of plants; but you might choose duckweed over that simply because you like
duckweed! (And don't mind removing excess regularly, or that it may take some work to completely get rid of if you change your mind later.)