Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Posts: 11,721
Before you add any fertilizer check the NO3. This will be all from fish food. See if it builds up or drops through the week between water changes.
If the level is zero or close to it, then the plants are using pretty much all the fish food. You could follow the estimative index or any other fertilizer program, without any adjustment.
If the level is low, or fluctuating, then the fish food is supplying a certain amount of nutrients, so you probably want to follow a modified program.
If the level is high enough to need water changes to keep it low (under 20ppm is a good target) then the fertilizer program would be greatly altered.
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Fish food supplies reasonable amounts of:
N, P, most traces.
Fish food is low in:
K, Ca, Mg, Fe.
Water changes, with GH at least 3 German degrees of hardness supplies Ca and Mg. If the GH is less than that, or you suspect the GH in the water is just from Ca or just from Mg, you might want to supplement it with whichever is missing. This is not dependent on fish food. It depends on the tap water.
If the plants are using pretty much all the N, then they are also using all the other elements. Fertilize with all the macros and micros at 100% of whatever program you are following.
If the plants are using most but not all the NO3, and you need to supplement. Lets say you find that 50% of the dose suggested by your fertilizer program is needed to keep the NO3 in the range you want.
Then you should also dose:
50% of the suggested P, and traces.
100% of the suggested K, Fe.
If the plants are not using much of the NO3, and you are doing water changes to keep it low, then there is also an excess of P, and plenty of most traces.
Supplement with K, Fe.