[APD] Re: The why of the Barr method

["Jim Seidman" <js5 at seidman_net>]

Daniel Larsson quotes Dr. Ole Pedersen in saying:
"Can allelopathy be used to control algae growth in the aquarium? The
answer would be NO!"

I've read Dr. Pedersen's article before, and it doesn't make sense to me.
He says in the same paragraph:
"It is much more likely that this general observation is due to efficient
competition for resources (light, nitrogen, phosphorus and CO2) from the
higher aquarium plants and this may prevent the algae from ever getting a
hold in the tank."

If this explanation was correct many people would have tanks overrun with
algae. Algae need very little light. Many people keep their tanks amply
supplied with NO3, PO4, and CO2 all the time.

I neglected one of my tanks for a few weeks and let NO3 levels get to about
0.2 ppm. I'm only now recovering from the algae takeover and green water
that ensued. How I am fixing it? By dosing KNO3 and NaH2PO4. And it's
working great! If nutrient competition was the answer my fertilization
would make it worse, not better.

To Dr. Pedersen's suggestion of controlling algae by low fish count (i.e.
limited nutrients) and frequent water changes: I tried it, it didn't work.
I have never heard of it working for anyone else either.

I also notice that Dr. Pedersen mentions in the same article that there are
plants that pump out quite large amounts of allelopathic toxins "under
natural circumstances." My theory is that this effect ratchets up an order
of magnitude under high-nutrient conditions.

Again, allelopathy explains the observed fact that high-nutrient aquaria
are often algae free. Dr. Pedersen's theory of nutrient competition
wouldn't predict that result.

- Jim Seidman


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