Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish food can also supply most nutrients, especially for a low tech tank. It is low in potassium and iron.
KNO3 is a reasonable source of potassium, but also provides nitrogen.
If your NO3 from fish food is already in the range you want, your plants will highly likely need a different source of potassium.
This is why K2SO4 is suggested for some tanks.
Magnesium may be needed, but I would look into it, first.
1) What is the GH of the tap water, and the tank? If the GH is over about 3 German degrees of hardness then the calcium and magnesium are likely in the right range for the plants. No need to add more.
2) If the GH is less than 3 degrees, then dose a more complete GH booster, which contains both calcium and magnesium in the right ratios. Plants use these minerals in a ratio something close to 4 parts Ca: 1 part Mg. The water does not have to have this exact ratio, but something close is a reasonable target.
If you are keeping hard water fish and need to raise the GH then use GH booster, not just magnesium.
3) If you suspect that the calcium and magnesium are not in a reasonable ratio, then test for calcium. Then make sure you get the correct formula to figure out how much magnesium there is. (it is not simply GH - Ca = Mg) Then you can dose whichever the tank needs in the right amount.