Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
It is not about the brand name of fertilizer, it is about the active ingredients.
Plants need over a dozen elements to grow. Some are supplied in the water, and fish food may supply many of them, but perhaps not in great enough quantity.
Plants use the most:
Hydrogen (H) and Oxygen (O). If your tank lacks these there is something else going on!
Carbon (C) is a tricky one. Most plants need CO2, but can handle a few other sources like Seachem Flourish Excel. High tech tanks usually use pressurized CO2.
Next most is a group of 3 elements that are referred to as Macros:
Nitrogen (N), Phosphorus (P), Potassium (K).
Fish food may supply N and P, but not much K.
Seachem bottles N, P and K separately as part of their Flourish line.
You can buy agricultural chemicals, dry salts, to supply these to the aquarium plants.
KNO3 supplies nitrogen and potassium. If fish food keeps the NO3 up pretty well do not use this.
KH2PO3 supplies phosphorus. Although there is potassium in it, this is dosed at fairly low levels, so it just about does not count for potassium. If the fish food seems to supply enough nitrogen, then it probably is supplying enough phosphorus, too, so do not use this if the NO3 test shows there is enough nitrogen and you know it is coming from fish food.
K2SO4 supplies potassium. If you use less or no KNO3 (because fish food supplies the NO3) then use K2SO4 for potassium.
Plants use less of these secondary nutrients:
Calcium and Magnesium are usually found in tap water as long as the GH is over about 3 German degrees of hardness. If you use RO, distilled, rain water, or otherwise lack Ca or Mg, then a GH booster is a good product. It will have Ca and Mg in the right ratio for the plants. Seachem Equilibrium is one such product.
Iron is a mineral that plants use a fair amount of, and is lacking in fish food. You can dose iron from the Seachem Flourish line, or get chelated iron powder, or get iron as part of a blend of trace minerals.
Plants use the least:
Trace minerals or Micros.
This is a group of about a dozen minerals that plants use in very small amounts.
You can dose these as Flourish Comprehensive, or a dry blend such as CSM+B. A trace or micros supplement will usually list iron, but many planted tank people find this is not enough iron, so will add more iron. Fish food supplies these, too.
If the tank is run as a low tech tank, low light, then you will probably dose this way:
Fish food supplies enough of almost everything except K, Ca, Fe.
You get Ca from tap water.
Dose K, Fe, and it would probably help to add do-it-yourself CO2 or a liquid carbon source like Excel.
If the tank is run with higher light, then you will probably dose this way:
Estimative Index, or other method that supplies all the nutrients that are not already supplied by the water.
That is, Ca and Mg are usually in the tap water, but all the other things are added to the tank.
Carbon as pressurized CO2.
N, P, K, Fe and trace minerals are added in some form or other.