CO2 system with low consumption

I am new to this mailing list and keeping aquatic plants, but thought that
I would explain my experiences so far with CO2.  I am living in Japan where
it is apparently hard to get big CO2 tanks and refills at reasonable cost. 
The local fish stores, however, are stuffed with high-tech German and
Japanese CO2 goodies that are hard to find in America.  The cheapest refill
tanks cost the equivalent of $15 per mini-tank of CO2 (60grams).  (Since
this IS Japan, the concept of "rip-off" is simply not applicable.  Japanese
pay for form, not function, but that's a whole different story.)  I didn't
want to mess around with DIY yeast explosions, so I bought a needle valve
for a mini-tank.  There is no pressure regulator, just a valve on a tank.

Since I'm on a limited monthly budget for aquaculture, my challenge was how
to create a low consumption CO2 system that would use less than 60 grams of
CO2 per month.  First, the vital statistics:  56 liter tank, one 20 watt
regular flourescent bulb, one 20 watt plant flourescent bulb, lights on 10
hours a day, water changes 25% once every two weeks, water hardness 4
degrees GH, "Nach" undergravel fertilizer tabs and Tetra iron intensive
fertilizer with each water change, 10 Neon tetras, 2 dwarf gouramis, 2
golden algae eaters (don't know what type), 2 siamese algae eaters.  The
plants pretty much fill up the tank and include Hygrophila, Rotala, and
Glassostigma.  The algae seems to be under control and the plants need to
be trimmed about once every two weeks.

Now for the saga of CO2 diffusion systems.  First off, I bought a gadget
popular in Japan that makes "pollen" sized CO2 bubbles with a very fine
glass frit airstone.  With this, most of the bubbles seemed to escape into
the atmosphere despite the manufacturer's claim of high efficiency.  I
suppose it was better than most regular airstones.  In order to lower the
pH from it's "natural" value of 7.8 down to the desired 7.0, I had to use
about one 60 gram CO2 tank every two weeks.

Next, I tried a Tetra diffusion chamber with about 4 cm2 of open surface
area on the bottom and a gauze-covered top with another 4cm2 of surface
area.  This was not able to keep my pH at 7.0.  There was simply not enough
diffusion area.  Oxygen bubbles from the plants tended to get in the
diffusion system and appeared to destroy it's diffusion ability.

Still unsatisfied, I launched into unknown territory.  I made a number of
different diffusion systems with my trusty aquarium cement and thin (PET)
plastic sheet.  Only one actually worked.  It was constructed to cover
about one third of the top of the aquarium water.  It is about 1 centimeter
high and fills the aquarium almost from the left side to the right side. 
The lights are in the back of the aquarium (over the plants), so the
diffuser is mounted in the front of the aquarium.  In addition to a large
surface area, it also reduces the surface area available to loose CO2 by
diffusion into the atmosphere.  I also had to reduce the trickle filter
operation, because it was loosing too much CO2.  I run the trickle filter
15 minutes every 6 hours (to avoid drying out completely and killing the
bacteria).  I also run it an hour and a half in the morning to help avoid
low pH in the morning.  By tuning the flow of CO2 AND the filter operation,
the pH can be kept nearly constant.

I found that the even though most of the oxygen bubbles from the plants
rise in the back of the fishtank, some enter into the diffusion system. 
Maybe air bubbles find their way in also.  Gradually, the diffusion system
fills until over a period of about four or five days, the entire system is
covered with trapped gas.  The pH stays constant even though the amount of
gas in the system changes.  This may be because the increasing oxygen (or
other gas) level is balanced by the increasing surface area.  The amount of
CO2 entering the water seems to be constant.  When the system becomes full
of gas, the pH starts to rise, so I dump out the gas and let it gradually
fill up again.  I end up using one 60 gram CO2 tank about every two months.

I have a few questions for you plant-wise folks out there in
aquacultural-land.  Is it safe to reduce the surface area of the fishtank? 
(The trapped oxygen in the diffusion system should help.  I never see any
signs of oxygen starvation in the fish.)  Will the filter still be able to
process ammonia/nitrites with such a limited amount of operation?  Will the
plants still be able to grow maximally even though the Dupla regimen of
constant water motion is not being followed?  Is this entire idea totally
crazy and useless?

Barret Lippey
Technology Development Department
Production Technology Division
Sony Corporation, Japan
Latest e-mail address: lippey at tft_ptg.sony.co.jp
Permanent e-mail address: blip at alumni_caltech.edu
World Wide Web: http://alumni.caltech.edu/~blip/
FAX:  0462-27-2313
Telephone:  0462-27-2315