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Re: Nitrates

Shireen Gonzaga <whimbrel at comcast_net> wrote:

>It's packed with mostly fast-growing plants. I have two
>aquaclear filters running in each tank, and I'm using twice
>the recommended filtration per gallon to ensure good
>biological filtration.

I wonder if this is part of the problem. I remember Diana Walstad
discussing this in "Ecology of the Planted Aquarium." It's harder for
plants to utilize nitrates than ammonia. To avoid using chemistry terms,
think of ammonia as having more energy, which is why bacteria consume
it. For the plants to utilize nitrate, they need to convert the nitrogen
back to the ammonia form, which is energy-intensive. Many plants won't
even make the enzyme to do this if there's ammonia present.

So she actually warns against having too much biological filtration,
saying this can lead to higher nitrate levels. In effect, the filter
bacteria are competing with the plants for the ammonia. So you're
converting a lot of ammonia into nitrates in the filter, and maybe the
plants can't convert all of it back.

In my tank I have no filter at all, and never got any ammonia or nitrite
spike. My nitrate levels keep going to zero. So I'm starting to think
that biological filtration is overrated for a densely planted tank. Your
approach of using twice what you need may be counterproductive.

>> On the other hand, if you add CO2, some of that CO2 will react
>> with water to form carbonic acid. (CO2 + H2O <--> H2CO3, meaning
>> that CO2 and water turn into carbonic acid and vice versa.) So
>> adding CO2 will lower your pH. But the more bicarbonate you add,
>> the less effect on your pH adding the same amount of CO2 will have.
>I was following you till that last sentence. I'm not quite sure
>what that means. Are you saying that when there's more bicarbonate
>in the water (higher kH), the CO2 is less effective in lowering
>the pH (making carbonic acid)? Or are you referring to the fact
>that pH is a logarithmic scale? Sorry, but I'm totally chemistry-

I meant that if there's more bicarbonate in the water, the CO2 is less
effective at reducing pH.

Think of it this way: there's some ratio in your water between the
concentrations of carbonic acid (H2CO3) and bicarbonate (HCO3-). In a
practical sense, this ratio determines your tank's pH. (I'm
oversimplifying a bit here, hence the "practical" comment. The presence
of other buffers, chemicals, etc. will change this.)

Remember that CO2 in water will form H2CO3. Now, adding a certain amount
of CO2 will increase the amount of H2CO3, thereby changing the ratio
between H2CO3 and HCO3-, and lowering your pH.

However, the more HCO3- that is in the water, the more additional H2CO3
necessary to change the ratio by the same amount. If you really add a
huge amount of HCO3-, then the concentration of H2CO3 you can create
through CO2 injection will be miniscule in comparison, and it'll be
difficult to budge your pH from 8.4.

- Jim