CHALLENGES.
The beauty of our hobby is that there is always a calling to take up just one more challenge. You know how it goes, at 5 years old we might get that gold fish in a bowl, then comes the 10 gallon tank of tropicals, a bigger tank to house all those free guppies, the monster tank in the bedroom room and so it goes on. Money of course is no obstacle! Then one day you think to yourselve, wouldn't it be nice to have one of those DUTCH tanks in the middle of the living room. You know, the type filled up with healthy looking plants, and aqua-scaped to look like a million bucks. And after a few enquiries you realise that all that special equipment and other stuff that drives such a master piece would actually cost a million bucks . Well the good news is that you can have your cake and eat it, but you do have to take up a few DIY (Do It Yourself) challenges along the way. For one thing, you will need to develop some basic understanding of biological and chemical processes. Another is to think creatively so that you can develop the required gadgets and methods, as well as zero in on various readily available cheap chemicals that are going to help foster a good plant environment. Contents.

THE NEXT STEP.
Is not for me to deprive you good readers from the above challenges, cause that would be like really mean. Wouldn't It! But....... the end of this article is not yet in sight as its title implies some 'talk' about CO2 in the aquarium, and I do actually feel like writing something more substantial. Contents.

PHOTOSYNTHESIS.
One of the key elements of a Dutch tank is the infusion of CO2 into the water for an improved plant environment. Healthy plants actively conduct the process of photosynthesis in the presence of adequate amounts of light, heat and CO2 (water is also needed and should be plentiful in an aquarium). Photosynthesis produces a type of sugar (hexose) and liberates oxygen. Sugar in turn is the energy source used by the plant to metabolise various nutrients for growth. It goes without saying that if we do not provide enough CO2 then the plants have limited growth potential. Also worth mentioning at this point is that plants will do anything to get CO2, even by extracting it from the carbonate hardness agents that are dissolved in water (more about this later). Contents.

CO2 IN WATER.
Properly infused CO2 will dissolve into the aquarium water (H2O), and a small amount of this dissolved CO2 will react with water to form Carbonic Acid (H2CO3).

The formula for this reaction is: CO2 + H2O = H2CO3.

In turn a small amount of the carbonic acid will dissociate (split apart) into its ionic components, a proton (H+) and a bicarbonate (HCO3-). The increase of protons (hydrogen ions) in water causes the Ph of the water to drop (ie makes it more acidic). Thats not the end though because a small amount of the bicarbonate ions also dissociate to result in yet another proton and a carbonate ion (CO3--). Once again the increase in protons affects a Ph drop, and thus the bicarbonate ion behaves as an acid. If we remove any CO2 then the amount of bicarbonates and carbonic acid must also reduce, this reverses the dissociations that took place and hence the Ph of the water increases. Since photosynthesis removes CO2 from water it is clear from the above discussion that the Ph of the water has a tendency to rise during the day (photo-period). Infusing CO2 into the water at the same rate as its removal by photosynthesis means no change in Ph, and that is good for plants and fish.
The relationships that dictates the extent of acid dissociation that takes place is complex and involves the carbonate buffering system (future article if there is interest in this). For now it suffices to say that the amount of dissolved CO2 is related to the carbonate hardness (KH) and the Ph of the water. Carbonate hardness is simply a measure of the alkalinity of water, and alkalinity is simply the ability to neutralise acids. Various aquarium books (eg the optimum aquarium) provide tables that allow you to determine the amount of dissolved CO2 based on the Ph and carbonate hardness of the water. If you are any good at mathematics then you can use this formula to caculate the amount of dissolved CO2.

Carbon dioxide in parts per million (ppm) is: CO2 = 3*KH*10^(7-Ph). Contents.

LACK OF CO2 IN WATER.
Without infusion there is a real possibillity that plants will deplete all of the dissolved CO2 in the aquarium water during the photo period. When this happens some plants will extract its carbon needs from dissolved bicarbonates. This reaction is called biogenic decalcification and effectively breaks down (removes) the bicarbonates dissolved in the aquarium water. Since bicarbonates are acids (see above) then its removal causes an increase in Ph of the aquarium water. The bicarbonates that are involved in this process must come from a bicarbonate salt, since there is no free CO2 and hence no carbonic acid to dissociate into a bicarbonate (got that). The bicarbonate salt is mostly calcium bicarbonate Ca (HCO3)2, the agent that makes water hard (temporary hardness).

The formula for biogenic decalcification is: Ca (HCO3)2 = CaCO3 + CO2 + H2O.

Since calcium carbonate CaCO3 is insoluble, it can be seen from the above formula that biogenic decalcification results in a calcium carbonate precipitate (white stuff on the plants). The CO2 produced is of course used to satisfy the biological needs of the plants as decribed above.
If you're happy with the above explanations then do not read the following paragraph but skip right to the next section on 'DIY Carbon Dioxide System'. Contents.

WARNING! WARNING! ----- DONT READ THE NEXT PARAGRAPH.

NEXT PARAGRAPH.
If you have turned to this paragraph then you must be a sucker for punishment, or you have been saying to yourselve "this chap tells crap". Crap because everyone knows that to increase the Ph of the water you add a bicarbonate salt (eg baking soda), so removing a bicarbonate salt should decrease the Ph. Well, you're right ..............and so am I. This is because a bicarbonate is an amphiprotic substance ie, it can behave either as an acid or a base. Which way it behaves depends on whether it needs to react with an acid or a base. If you want to increase the Ph it must mean the aquarium water has an excess acid base (usually carbonic acid, right!). In the situation of biogenic decalcification there are no carbonic acids left and the Ph is already quite high. Want more on this caper .....................................DIY. Contents.

DIY CARBON DIOXIDE SYSTEM.
A very easy way to infuse CO2 into water is by bringing it into contact with water over a fixed surface. A 40 gallon aquarium with a water KH of approximately 4 degrees (70 PPM ) is satisfied with 20 to 30 cm2 of CO2 surface. Use upside down jam jars or tall drinking glasses to create a CO2 gas bell that lasts a few hours. An aesthetically more pleasing bell can be made from pieces of window glass siliconed together. Contents.

FIRST PART OF THE DIY CO2 SYSTEM.
Make a bell jar about 15 cm deep (this will lasts for up to 10 hours, and is perfect for one photo period). For larger aquariums, or if the KH of the water is higher, the mouth of the bell needs to be increased as per table below (surface area required is not that exacting).

Carbonate Hardnes (KH) in German Degrees. Contents.

CARBON DIOXIDE SUPPLY.
Various methods to obtain CO2 are described in aquarium books. However none of these appeal to me as they are either to messy, to bulky, to costly, or just a proverbial pain in the butt. As a DIY person I am always on the look out for the perfect CO2 solution. One day (many years ago) I struck it rich when my attention crossed over to those Fizzy Drink Makers (FDM). Surely I thought, this contraption is the answer to my prayers. And just to confirm its potential I proceeded to pull one appart, right there in the shop (afterall I was going to buy it, maybe). Fig 1. and Fig 2. shows what the FDM looks like.
The soda stream FDM comes complete with a 250 gram (net) CO2 gas bottle. This gives you a whopping 125 liters of CO2 gas, enough bell jar refils to last at least 6 months if you are a bit generous on bell jar refils. Empty CO2 bottles can be exchanged for recharged units at Kmart or Big W for about $6.00. Second hand soda stream FDM (complete with an empty gas bottle) are regularly available at Cash Converters for between $10 to $25.
Normally, to make a fizzy drink, the glass lemonade bottle (3/4 filled with water) is pressed against the rubber seal by pulling the handle on the FDM forward. A firm press on the FDM carbonating button charges the lemonade bottle with CO2 (the fizz in the drink). If you over charge the lemonade bottle, the pressure relief valve operates to discharge the excess gas pressure (this prevents the lemonade bottle from blowing up). The pressure relief valve is connected by means of a clear plastic tube that runs along side the CO2 gas bottle. This plastic tube provides a convenient means to tap into the mechanics of the FDM. Contents.

SECOND PART OF THE DIY CO2 SYSTEM.
Purchase a soda stream FDM and convert it for aquarium use as described next.
First, cut a 6 mm hole in the back cover on the side where the plastic tube is situated.
Second, cut the plastic tube at the height where you have drilled the hole and fit in a plastic T piece. Suitable plastic T pieces are the type with barbed ends (eg the kind used for micro irrigation systems) and are available at any plant nursery or Kmart.
Third, slip on a piece aquarium air hose onto the remaining free end of the T piece.
Fourth, secure all of the T piece hose connections with plastic cable ties (of course you get those at Kmart's automotive section).
Finally, slip the air hose through the hole in the back cover and put the cover back on the FDM. Contents.

THIRD PART OF THE DIY CO2 SYSTEM.
Having converted the soda stream FDM and constructed a bell jar, I can assure you all that the worst is over (truly). All that is really left to do is to hook the air hose from the FDM to the bell jar via a check valve and purging tap. Fig3. shows the complete CO2 system in schematic form. Take note of the way things are placed, such as the orientation of the check valve, securing the bell jar in the aquarium with its mouth below the water line, bringing the CO2 feeder hose all the way to the top of the bell jar. Now lock in an empty bottle in the FDM and don't forget to place the operating handle in the working position as per Fig 4. (you be amazed how many calls I have had on this).
Note 1:
CO2 will draw water up the feeder hose when the bell jar is full of water. A reliable check valve fitted in line with the feeder hose will prevent this back siphoning. I recommend the type with the spring loaded neoprene (black rubber) bung. Other types may not close completely with little back pressure, or tend to disintegrate over time.
Note 2:
Never ever remove the Pressure Relief Valve of the FDM, its there for safety reasons. Contents.

DIY SYSTEM IN USE.
This is simplicity itself. Every morning give the carbonating button a little push to fill up the bell jar. Over time some ait will get in the bell jar and this progressively reduces its CO2 holding capacity. When the bell jar has to much air in it, just let it escape by opening the purging tap (clamp on hose works great). Thats all "who hoe". Contents.

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