Thanks Darrel, much appreciated looks like I need 4.5g for my 180L tank. Just for my own education, as above, "if our fertiliser has 20,000 ppm of Fe. If we take 1g of "Chempak sequestered iron" and add it to 1 litre of water, we've divided 20,000/1000, so we have 20 ppm Fe" ..... why do we...