The cheapest way is if you or Mr S. are feeling creative you could make your own. There is a thread or article here somewhere that gives instructions, I will try to find it.
Secondly you can get cheap commercial versions, such as
Tetra-Plants CO2-Optimat which uses a pressurised canister, Tube and the Diffuser in the tank. With this one you have to press the nozzle on the cannister everyday when the lights of the tank are on in order to fill the diffuser. This one is hard to control the amount of CO2 entering the water. It is large enough for a tank up to 100litre. It takes approximately 8 hours to diffuse contents into water.
Hagen CO2 Natural Plant System. This one uses a canister into which you put sugar and warm water plus the yeast and stabilizer that Hagen supply. This then has a tube connected to it that runs into the tank and connects to the diffuser in the water. Again this type is hard to control the amount of CO2 entering the water. Another drawback with this one is that it keeps running for 24 hours so you do need to add your own clip to the tube to stop CO2 entering the water when the lights are off as plants do not dissolve CO2 in the dark, and you will get a CO2 spike and this will drop your PH considerably. But by adding your own clip to the tube you can stop this from happening.
Thirdly you can get something like the JBL Pro-Flora 602. Although a lot more expensive at Around ?180, it is by far the easiest to use. I have put a link to JBL's site for this below to show what it contains. But in essence it consists of a pressurised bottle that has a regulator connected to it. Long story short when you have it set up you can regulate the amount of CO2 using the supplied bubble counter. If you connect it to a timer switch you can regulate how long the CO2 unit supplies CO2 to the water via a small electrical device called a solenoid. Once it is all up and running all you need do is change the bottle every 3 or 4 months or maybe more for your tank. The dials on the regulator let you see when the bottle requires refilling. Some shops are actually giving away a spare bottle free, (usual cost about ?89), so that you can always have a full bottle to hand. To refill it costs about ?18 pound. Compare this as follows.
Tetra canister = ?6.99/month Hagen Yeast and stabiliser = ?6.50/month JBL refill = ?18/3 or 4 months Home made incalculable as it depends on your own makeup.
In the long run the JBL style system does work out cheaper and is much more precise. The thing to remember here is that CO2 does not only help the plants but it also lowers PH and hardness. Not too bad a thing if you are keeping Angels but can be disastrous if you are keeping Guppies etc that need a high PH.
All above figures are approx and other manufacturers do make similar units, but JBL is what I use. I have also upgraded the bottle from the supplied 500g to the 3.5kg bottle due to size factors of my tanks.
I use a homemade CO2 system which is really cheap.
1. 2 Cups of Sugar 2. 1/4-1 teaspoon of yeast (more yeast = faster reaction = more CO2 for shorter time (2+ weeks)) 3. 1/4 teaspoon of baking soda (optional)
this is needed for each bottle
2x 2 litre fizzy drinks bottles (with caps) silicon tubing 1x T conector for tubing CO2 diffuser something like this one way check valve silicon to glue tubing in place
Drill a hole in each of the fizzy bottle caps just smaller than the diameter of the tubeing, then cut 2 lengths of tubing about 30cm long and insert one into each drilled cap, and then using silicon sealer put a bead around the inside and outside of the cap to seal the tubing. Then let it set for 24hrs.
After it has set connetc the T conector to the ends of the tubing, then cut a 30cm peice of tubing and conect one end to the T conector and the other end to the one way check valve (make sure its the right way round) then conect the length of tubing to reach the desired point in the aquarium and add the diffuser. Fix in place, your then ready for the recipe.
Adding the recipe
add 2 cups of sugar for each bottle, and then fill 3/4 full of warm water (not hot as it will kill the yeast), add yeast and baking soda (if using) screw on lids don't shake or mix it may take 24 hrs to start to get any production of CO2.
You can add a bubble counter too it as well which will stop any yeast gunk getting in to the aquarium but I've not had that problem.
I'd definitely add a bubble-counter/gravity trap to any yeast based system. You can make one from a small pop bottle.
Yeast based CO2 is high maintenance and can lead to varying CO2 levels which in turn results in algae and sporadic growth. Not to mention fluctuating PH levels - although within limits (0-30ppm CO2) the PH variance caused by CO2 levels is not harmful to fish.
I would not recommend spending money on a commercial yeast based system if money is tight. They really are little more than a bottle and length of pipe.
For a pressurised system I would heartily recommend a PH controller rather than a timer - although this adds another ?40-?140 to the cost of the system it allows precise CO2 levels to be maintained. I've noticed that occasionally my regulator shifts from 30 bubbles a minute to something closer to 3000. Without the PH controller the tanks PH would dive and my CO2 would run out very quickly. The valve also sometimes closes on its own, but then I see the controller measuring ph 6.9 rather than 6.8 and I take a look in the cupboard to see whats wrong. (Of course I could buy a better regulator too)
From what you said about budget I'm guessing the pressurised CO2 kit is out of reach at the moment - keep an eye on ebay for bargains.
This was my yeast based system before I moved to a pressurised kit.
Hi Mike how do you regulate the amount of CO2 entering the tank, and how do you stop it from flowing when the lights are off as this is important, due to the fact that plants do not dissolve CO2 in the dark, and that a regulator is required to stop your PH being lowered too much. I have actually got my PH down to 4 using CO2, (without the fish in the tank). If you start with hardwater this is not so bad but if like me you start with a ph of 6.8 the Ph can fall drastically if it is not controlled properly thus the need for a bubble counter and tap to stop the flow when you turn off the lights.
edit// sorry crossed with EagleC. But I agree with Eagle as I to have a PH controller, but I was trying to keep costs down to a minimum for Christina. The 602 does come with a PH monitor that goes in the tank and registers the PH by colour.
You can't turn off a yeast based solution without risking the bottles exploding (this is bad).
You can set an airpump to run, and this could be just at night but running it 24/7 will also work. Although you will waste 60% of the generated CO2 during the day you will keep the PH and O2 levels more constant.
I worked out a way to regulate yeast based CO2 without risking an explosion but it requires an airpump, PH controller and a solenoid. Bought separately these items are nearly as expensive as a budget pressurised kit from ebay!
No you dont have it running all the time. You only need to add CO2 during the hours the lights are on as plants do not dissolve any Co2 in the dark. You will need a way of shutting off the tubes at night so as no CO2 gets in your tank as this would play havoc with your PH.
EDit// Sorry Eagle, did it again. I didn't want to mention the blowing up bit... thanks for doing that bit
My Ph seems to stay at a constant 7.6 and and i have very hard water where I live, and have a JBL ph monitor that is stuck to the glass http://www.jbl.de/factmanager/index.php?lang=en. I do use a clamp to restrict the flow during the night and release it in the morning the presure doesn't seem to get too high as it's not shut completely so tiny amount does enter the tank (use a bubble counter to check this), I have thought about having another tube coming off as a release system. If I could afford a high tech system i would but I have no loacal suppliers that supply CO2 bottles and sending away for them would cost me to much.