Most dry ferts are just micros. You typically have to add Nitrate, Phosphate, and Potassium (K) seperately. I believe this is because some of those chemicals react with iron making it un-usable to the plants. This is also why you need to use deionized water if you want to make a liquid fert from it. Because your tap may contain enough chemical to drop the iron out of solution.

Also check the water you are adding or look at the city's water records if you are using tap water. In my area there is soooo much phosphate already in the tap that I never have to add any. I also have enough fish to keep my nitrates right around 10ppm so I don't add that either. I just add Potassium and the micros.

I guess it depends some what on the dry "pre mix" you use.

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--->Pictures of my 75g Amazon tank<----