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[APD] RE: Target N/P/K levels

Benjamin Hong asked: "At what levels should N/P/K be kept?"

We should distinguish between the kinds of N and P.

I think this depends upon several factors including light level, CO2
level, presence & type of filament algae and the amount of pruning
maintenance desired. Let's assume that you don't have a serious
infestation of Spirogyra, Oedogonium or Cladophora or other of the major
categories of filament algae and that you are interested in strong
growth. If you are changing water every week then a good target at the
start of the week might be:

Nitrogen: 5ppm nitrate and <0.05 ppm ammonia
Phosphorus: mineralized phosphate 0.1 ppm (I defer to Tom) organic
phosphate << 0.01
Potassium: 10-40 ppm (in mineral form, is there any other kind?)

I would suggest targeting a lower nitrate level if you are willing to do
more frequent water changes. If you only change or dose every couple of
weeks, you might shoot for a 10 ppm dose of nitrate.

If you're going low-tech with about 1 wpg or less and no external CO2
addition, you might target 1 ppm of nitrate or rely upon the fish and
fish food to supply the N & P requirements.

Personally, I haven't yet had the guts to add phosphate to my tanks
except in clay ball form. I typically use soil with peat as a substrate
and amend it with clay fertilizer balls. I'm sure there is a substantial
amount of P getting into my water via leakage from plants, fish, fish
food and decaying poop from snails and fish. I should like to know more
about phosphorus/phosphate uptake since Tom has talked about algae being
able to utilize organic phosphate complexes that macrophytes don't use.
Suffice to say that algae conditions in the aquarium generally benefit
from frequent water changes to reduce organic phosphate and other

Thinking of what Tom has said about macrophytes preferring mineralized
phosphate, I'm wondering if it wouldn't be good to supply this using
clay balls in lesser amounts but more frequently. This tends to keep a
higher concentration inside the clay ball for preferential root uptake.
I'm not convinced at all that increasing nitrate & phosphate in the
water do not cause algae to grow faster. In other words, I believe that
if you increase them in the water you will stimulate algae growth. Of
course, when the macrophytes are growing well, they sprout new leaves
and may keep ahead of epiphytic algae and also shade out algae at lower
levels. New leaves also lets you prune out the old algae covered leaves.
We humans, and our algae eating pets, are agents in the competition


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