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Re: Tank Life or substrate nutrients and CO2 required?

Michael D Nielsen <mnielsen at U_Arizona.EDU> writes:
> All the talk about adding soil/peat/other organics to your substrate
> seems like it will only last a short time before the need arises to 
> replace the material used by the plants. Is this the case?

This depends upon how you define a short time. Relative to other
methods, using substrate fertilization requires the least frequent

You may need to add nitrogen + phosphorus fertilizer to a substrate
about every 6 months to a year if you have high lighting and a lot of
growth. What happens is that growth rates simply decline over an
extended period of time. If you have lower light you can rely upon the
fish and fish food for macro nutrients so you may never need to renew
the substrate. It is very useful to supplement sword plants to get large
and luxuriant growth. Of course it depends upon how large your tank is.
Maybe you don't want an amazon sword plant with 30 inch leaves. ;-) 

In terms of iron (and other micro nutrients), you will never need to add
iron to a soil substrate. Peat is used because it is extremely slow to
decompose; possibly hundreds of years in. Peat maintains a sufficiently
low redox potential to maintain consistent iron availability. Ordinary
substrates can become too well oxygenated by root systems after a period
of time according to Paul Krombholz's experiences. 

Ordinary soil is not an organic material although it may contain a
certain percentage of humus which is organic in origin or other fibrous
organic components. Garden soil is primarily mineral. Potting soil,
compost and other materials are much higher in organic content and may
decay at a higher rate (more labile) than we desire releasing more
nutrients than the plants are able to cope with. This is a case where
more is definitely not better.

If you do not have a source of N or P nutrients in the substrate then
you are relying upon sources from the fish excrement or upon
daily/weekly additions of chemicals such as PMDD. Whatever the system,
you need to provide a balance between nutrient and mineral inputs and
plant growth. Using the substrate for this has certain advantages and
requires fewer chemical tests to maintain stability.

> I would like a method which can be set up for numerous years (5+) without
> any need to tear down the system to replace the substrate. Would
> laterite perform this function for a long enough time.

I don't think that any of us soil people have ever said that you need to
tear down a soil substrate every 5 years. This seems to be a rumour
started by the people who sell laterite. :-) You could use laterite
(which IS a soil) in combination with peat to provide a continuous
source of substrate iron but it is better to use ordinary soil which is
also a source of micro nutrients not present in laterite.

The Dupla system and other similar systems based on laterite are proven,
reliable, reproducible methods for growing aquatic plants. This system
is based upon moderately high light, CO2 injection and regular water
changes and regular doses of the Dupla chemicals. You need to follow all
aspects of "the system" to guarantee success.

> Another thought is, is anybody out there not using CO2?

Yes, definitely. We mainly use CO2 supplementation in tanks with high
light intensities. Low light tanks can grow plants very nicely without
it. Many people maintain low tech planted tanks; perhaps the majority of
planted tanks are in this category. 

I don't use CO2 on my killifish breeding tanks. I use fairly strong
lights and plants and rely upon peat containers in those tanks to
maintain stable pH. I've been using this method for about a year with
excellent results.

> While the expense is one problem I have also read that 
> certain fast growing plants end up
> looking bad in a CO2 system and thus shouldn't be used.

This might occur if the plants are competing for light and they are
growing tall rather than bushy. Using the yeast CO2 system is not
expensive but time consuming. I'd rather spend a few hundred bucks on a
CO2 tank, regulator and metering valves than fuss with the yeast

> Also a problem I have always seen is how is the water 
> for water changes brought to the same
> pH of the tank system with the CO2 bringing the pH down?

There are chemical pH buffering systems in your tank water which
maintain a fairly stable pH even if you are adding very alkaline water.
It's not a problem.

If you have a high light system without a significant CO2 input of some
sort, you will encounter a fairly rapid rise in pH caused by the plants
obtaining carbon from carbonate ions to perform photosynthesis. This may
not bother several types of plants but its not too healthy for the fish.
I use the humic acids released from peat to prevent this in my Killifish
tanks. The pH of those tanks is extremely stable. A neutral pH is said
to be important for producing an equal number of male to females in