If you plan on having plants in your aquarium then injecting CO2 into your tank is highly recommended. Having a yeast-based CO2 system for me is the easiest, cheapest and safest way to go (as opposed to a canister-based system). When using a yeast-based system, like this Hagen one, it's impossible to overdose CO2 in your tank which is nice.
Remember, having 2-4 watts of "quality" fluorescent lighting per gallon (compact fluorescent fixtures seems to me to be the best choice) in your aquarium is even more important than adding CO2.
This off the shelf system should be sufficient for beginning aquarists who are starting out with plants (fish don't need extra CO2). While not as efficient or cheap as a DIY one, it does a decent job of putting a fair amount of CO2 in the tank for about 2 week periods at a time. People who also don't want to mix their own yeast mixture will find the packets more convenient (however less effective).
The Hagen diffuser (that dissolves the CO2 bubbles into the water) does a decent job. However, there is waste, meaning, CO2 bubbles escape to the surface before being dissolved. But you still will get a raise in the tank's CO2 level.
So, if you are new to CO2 systems and want something that is minimal investment ($35) and don't want to make your own system, then this is a no brainer system. I think you should get it and help your plants to thrive.
But if you want a cheaper, much more effective system, then do this: For 10-25 gallon tank, use one 2 liter soda bottle. For 25-50, use 2. Mix 1/2 teaspoon of bakers yeast in warm water and then let it sit for 5-10 min. Then dissolve 2 cups of sugar into filtered water and pour into soda bottle. Add the yeast. Drill a hole in the soda bottle cap and either glue an air line to run through it or buy a metal airline connector (like from Lowes or Home Depot) and glue it (using Goop) into the cap. After you have the air line running out from the soda bottle (if you have two then connect them using a T brass connector), run it into your aquarium and dissolve the CO2 using a diffuser (like this Hagen one, howbeit not efficient), an air stone (also not efficient) or into a small pop bottle that a Rio Power Head is forcing water into to completely dissolve the CO2 bubbles. You can search around and find photos on how this is done.
After all of my trial and errors, the above method using soda bottles with a metal airline connector in the cap (glued in with goop) and a Rio Power head is the best, cheapest and most effective system for pumping CO2 into your tank.
If you are purchasing this Hagen system, just remember that for every 20 gallons of water you have in your aquarium, you need to set up another Hagen canister. So for a 40 gallon tank, buy 2 of these. But remember, you will get a nice rise in CO2 levels but not near the level you would using a forced water reactor using a Power head. There are two parts to a CO2 system: the amount and quality of CO2 bubbles being produced and then also the amount those CO2 bubbles are being diffused into the aquarium water. The best is having the bubbles completely disappear into the water. Then you know there is no waste to your CO2 production.
To find out how much CO2 is effectively being dissolved into your tank, search online for the CO2 measurement calculator. This is measured by the ratio of KH and PH in your tank (as long as you haven't used PH buffers).
Hope this helps you in making a valued purchase.
This system is simple to use and affordable compared to most CO2 systems, but if you're willing to put in a little effort and research you could build your own CO2 fermentation system that works just as well (if not better) for a fraction of the cost. Still, not everyone is willing to build their own, and this system is a decent compromise. It's also pretty safe, since the CO2 injection is slow and gradual and won't cause fast pH drops. Make sure you have a decent buffering capacity just in case.
My only criticism is the "activator" and "stabilizer" packets which Hagen sells for this product they are too expensive for what they are (basically yeast and baking powder) and a home-made mixture seems to work better for me anyway. With Hagen's mix, I couldn't get more than about 9 ppm CO2 in my 10 gallon tank (about 3 4 bubbles per minute) but a simple mixture of baker's yeast, sugar, and water gets me twice that and lasts just as long (2-3 weeks). Most people aim for 15 25 ppm or so (but no higher).
In any case, if you have adequate light (2.5 watts per gallon or more), your plants will benefit from CO2 injection and Hagen's system does what it advertises.
One tip: buy a simple air valve for the tubing so you don't get a siphoning mess when you're re-filling the CO2 cansiter.
Buy Hagen Nutrafin CO2 Natural Plant System with CO2 Activator and Stabilizer Now
This product works well, producing CO2 relatively easily and cheaply compared to having a pressurized CO2 canister and regulator (etc.) high-tech set-up. Be sure to educate yourself about CO2 in fish tanks. It's good for plants, but too much could be bad for fish. For example, I operate an airstone when lights are out at night only. This diffuses the CO2, which is not needed by plants in the dark, and plants are not producing oxygen in the dark. (Put air pump and airstone on a timer.) You need to learn the basics of CO2 injection, even for this simple system. If you do, you will have no problems, only great plants.
I have only small fish (under 3 inches) in my 55 gal planted tank. Not one fish has ever suffered as a result of CO-2 fertilization, but the plants grow much, much better!
I see no reason why a DIY yeast+baking powder solution would not work -will be going to this. For the price, you get all you need to get started, to make your plants look great!
Update: In the years since I wrote this, I have lost a couple small fish (which are typical of planted tanks) getting trapped between the diffuser and the glass. Over time, the rubber suction cups fail. That solved the problem for me fish can no longer get trapped in that small space. I now use plants of other times in tank to secure the diffuser into proper position. Still like the product, been using it for years. CO-2 is a big help with the plants, which help control nitrates, add beauty, make fish happier.
Update 2 (2012): I would still recommend this product for anyone starting out with CO2 to stimulate a planted aquarium it works! I still use the container that comes with this kit to generate CO2 with sugar, yeast, baking powder, and water. Don't bother with Hagen's expensive powders they are just yeast in one, and baking powder (not baking soda) in the other. Get it at the grocery store. One container of each will last for years, cheap! The CO2 generator in this kit seals well better I imagine than a DIY soda bottle with silicon would. But I no longer use "the ladder" CO2 diffuser that came with this kit. It did work well for years, other than a few fish trapped behind (usually during a water change hiding). Now, I am experimenting with a glass and ceramic disc diffuser, which works well, and Fluval (Hagen) ceramic disc in round plastic diffuser which is working well as also. Just plug into the airtube, wait 12-24 hours (for pressure to build) and they work great! Be sure to learn the basic of CO-2 injection in aquaria. Co-2 should lower the pH gradually, so you need to be sure your water has adequate buffering agents as well as watching the pH to ensure it doesn't get too low. A cheap easy way to increase buffering capacity and pH is Baking Soda (Sodium BiCarbonate). When adding it, do so slowly and gradually (not too much in one day) to avoid sickening the fish. I use Mardel 5-in-1 test strips, but have some way to test Nitrite, Nitrate, Buffering Capacity, and pH of the tank water.
Read Best Reviews of Hagen Nutrafin CO2 Natural Plant System with CO2 Activator and Stabilizer Here
I have had the Nutrafin system for a few months now. I used the included Activator/Stabilizer packets for the first 3 months,
then switched to molasses based sugar and yeast.
Some observations:
The ladder style CO2 diffuser really works well. Evidence of
this is the size of the CO2 bubble that is introduced into
the diffuser at the bottom compared to the size of the bubble
that comes out at the top. The bubble becomes an order of
magnitude smaller as it traverses the diffuserwhich is another
way of saying that CO2 is getting dissolved in the water. When
it is first introduced to a new aquarium, this observation alone could verify that it is working.
For anyone expecting quick results, it took about 15 days before
my plants showed "pearling" and about another week (around 21-22
days) before I saw the promised lush green plant growth. (Note
that I have 2.5 watts per gallon of light for 10 hours a day and
use Seachem Flourish once a week.)
There is no easy way to turn off CO2 diffusion with the system.
Once the lights turn off and unless the water is sufficiently buffered, pH can be dramatically and dangerously reduced. Start
with buffered, neutral water and after 3 weeks it may have turned
slightly acidicwhich is an ideal environment for many aquatic
plants.
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I have used this system for two years. It works pretty well. Hagen's yeast and 'stabilizer' last about a month in the winter, 2-3 weeks in summer, when the yeast seems to burn off quicker. I did once lose a fish that got trapped in the diffuser, but that fish had been looking unhealthy (inactive, not feeding) anyway. You can use bread yeast and baking soda instead of Hagen's expensive yeast, but it doesn't last as long.
The important thing is that CO2 is released into the tank and the plants look healthy. I don't bother to test my PH in my 20 gallon tank, so I don't know the effect on acidity..
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