Words by Nathan Hill 

Fluval Bio-CO2 Pro 250 review

Getting the right amount of carbon dioxide into a planted tank has been an aquascaping challenge since aquarists first added greenery to water. Now Fluval are tossing their ingenuitive hats into the gas-delivery ring with the new Bio-CO2 Pro low-pressure systems. Unlike the majority of high-pressure, pre-filled canisters of carbon dioxide connected to expensive regulators, these instead utilise natural fermentation—in this case, yeast and sugar. 
 
There are, in the UK at least, currently two sizes available: the Bio-CO2 Pro 125 rated for up to 125-litre aquaria, and the Bio-CO2 Pro 250 (which I'm testing here), rated for up to 250-litre aquaria. The US market also has access to a considerably larger 500-litre capacity model, though there are no early indications of this coming to the UK. I have the 250-litre model to play with.

Out of the box, you get the attractive, metal fermentation chamber with its screw-on lid, and a regulator to control CO2 outflow on the top of the lid. There’s also a bubble counter included which needs to be part filled with fluid and screwed on top of the regulator. A length of carbon dioxide resistant tubing comes as standard, along with an in-tank diffuser (plus the suckers to hold it in place in the tank), and a non-return valve. Inside the bubble counter you’ll also find a spare gasket for the connection between the regulator and counter. It’s tiny, so keep it somewhere safe. 
 
Lastly, there are the necessary packs of activator (yeast) and CO2 booster (which I suspect is a tablet of baking soda) to get the fermentation process started.
 
Fermentation to create CO2 isn’t a new concept; Fluval made a much smaller, plastic, uncontrollable, and much less attractive model some 20 years ago, before carbon dioxide injection had really taken off in the UK, though at that time it was largely viewed as a novelty rather than serious kit.
 
This newer design is less about novelty and more about form and function. The Bio-CO2 Pro has a distinct ‘ADA luxury’ look about it, and a capacity that laughs at its earlier iteration. This is a product aimed at the aspiring aquascaper, so how does it behave?

Setting up the Fluval Bio-CO2 Pro 250

Assembly and activation are reassuringly simple. Screw the bubble counter to the top of the regulator and attach the tubing to it, making sure to use the threaded union to fasten it into place, connect the diffuser to other end of the tubing with the non-return valve somewhere along the way, and sucker it into the tank. 

Though it doesn’t mention it explicitly, I’d leave the diffuser to soak for 24 hours before use. It’s not a major issue, it’s just that until soaked, the diffusor will release large bubbles, which isn’t what we want. 
 
To bring the canister to life and start producing gas, both packs of activator and both CO2 boosters then go into the canister (the lid simply unscrews for this), along with 500g of sugar (not supplied), and then you fill to the fill line (marked inside the canister) with room temperature water. 
 
Screw the lid back on, and 24 hours later you’ll have the first inklings of a CO2 supply. You can now control the flow of gas via the regulator to measure how much gas is being delivered through the bubble counter. 
 
It’s worth mentioning that unlike some high-pressure CO2 systems, there’s no solenoid built in to the Fluval Bio-CO2, so you’ll need to turn the regulator on and off daily if you don’t want to risk plunging the tank’s pH (not to mention a possible carbon dioxide overdose) overnight. 

 
In the event that the regulator remains closed for a prolonged period, there’s no danger of the cylinder expanding or bursting. There’s a release valve built into the lid if the pressure starts to exceed the fermentation chamber’s capacity. 

Fluval Bio-CO2 Pro 250 performance

My own test model began delivering gas after 24 hours, but there are some limitations. The most obvious of these is fermentation speed, which in turn is regulated by temperature. On one side of my room (I set the unit on the floor so that I can see it) the unit can sit in front of a radiator, and the gas production is markedly different to when it is sat the other side, next to a cold patio door. Warmth will dictate production rates of gas, but also the lifespan of the yeast, as it will burn through the fuel (sugar) far faster at a higher temperature. 
 
The other limitation is consistency. The Fluval BioCO2 is a low-pressure device, that runs at a maximum of 19psi, as opposed to pressurised cylinders that run at 800+psi. This means that once the regulator is opened and the unit starts feeding gas into the tank, the pressure level in the cylinder drops and the bubbles slow—the initial gas production in my case struggled to meet one bubble of gas per second. 
 
After some tweaking by opening the regulator every time flow declined, I found that the pressure kept on dropping and delivery became worse, and so I decided to open up the canister. Sure enough, I had a pool of sugar at the bottom, presumably with suffocating yeast underneath. 
 
With the lid reconnected and the regulator closed, I gave the whole thing a shake and in next to no time it was up to my required gassing speed, which suggests that the sugar might need to be in more suspension for the yeast to work on it. When it’s time to refill the unit with fresh activators and sugar, I’m going to make sure the sugar is dissolved in the water first, as I suspect this is the issue.
 
Since shaking, and at time of writing, the Bio-CO2 Pro 250 has been running for three weeks and delivery is now starting to drop off. At peak, I was receiving a consistent one bubble of gas per second throughout an eight-hour period, though this is now closer to one bubble every two seconds, suggesting that the first batch of sugar and activator is close to exhaustion. 
 
It’s also worth noting that not all of the gas released seems to be pure carbon dioxide, or if it is then my drop checker doesn’t read it in the same way. At one bubble per second from a high-pressure CO2 system, my drop checker becomes a lighter green colour than it does with one bubble a second from the Fluval canister. 

Fluval Bio-CO2 Pro 250 price and running costs

The initial outlay may be a sticking point for some. The Bio-CO2 Pro 250 isn’t cheap at a retail price point of £199.99 (with £159.99 for the 125-litre capacity model), though what the real ‘in store’ price will be I don’t know yet. But that does feel a lot for what you’re getting.
 
The saving grace is in the running costs. A refill pack carries an RRP of £12.99 (I’m expecting around £10 in stores) and that contains four activators and four boosters. A standard dose of two of each provides gas for about 30 days (or 21+ days and waning in my own case), so realistically it’s about a tenner for maybe 45-50 days… plus some sugar. 
 
You’d have to do your own maths on it, but in my case, I was previously using a high-pressure CO2 delivery system and getting through an 800g pressurised CO2 cylinder roughly every 90-100 days, which cost just over £45 to replace—the Bio-CO2 Pro has the potential to be a long-term money saver for me. 
 
Keepers of large, high-energy aquascapes might find the delivery too low or inconsistent for a tank full of hungry carpeting and stem plants—Fluval’s own recommendation for the 250 model is to start at 24 bubbles a minute, which feels low for carbon-hungry tanks. But for hobbyists who keep mid-energy layouts with more robust plant species (which, if I’m being honest, is most of us), and who are looking to improve plant growth without the need for intense gas injection, will definitely find the unit beneficial. 
 
Finally, if this design is using the same yeast and sugar formula as its older iteration, I’d heartily advise opening it somewhere well ventilated when it needs the contents replacing. What you ended up with back then was a curious form of ‘aquarist’s moonshine’ that had an aroma I still remember two decades on.

Is the Fluval Bio-CO2 Pro 250 worth buying?

Yes. Look past the initial expense and what you have is a gorgeous design on a relatively functional piece of kit. And all with the peace of mind that you don’t have a volatile, highly pressurised canister of gas sat under your tank, which will certainly reassure aquarists with inquisitive children in the house. I usually hide my CO2, but on this occasion it's out on show—just close to a radiator.

Why is carbon dioxide needed? 

Plants, whether aquatic or terrestrial, require three core ingredients to thrive: light, food and carbon. Of the carbon sources available to them, carbon dioxide, or CO2, is the easiest to uptake. But while there is some carbon dioxide found naturally in an aquarium, from the likes of fish respiration, or that which diffuses in from the air around the tank, the levels are usually very low. 

The easiest way to promote healthy photosynthesis in plants is to add an additional source of carbon dioxide gas directly to the water. This is usually done with a pressurised cylinder (either using a high-pressure cylinder like a fire extinguisher and a solenoid controlled regulator, or a low pressure system as reviewed here) of pure CO2 gas connected to a diffuser in the aquarium. The diffuser creates a fine mist of gas, and because this mist comprises of tiny bubbles that stay in the water for a long time, a great quantity of the carbon dioxide is dissolved into the water. Aquascapers with high-energy layouts (aquascapes that receive intense lighting and high level of nutrients) aim for a CO2 level of around 30ppm.

Gas content is usually measured with a drop checker. This clear device sits either inside or on the edge of the tank and contains a solution that reacts with carbon dioxide. If the level of CO2 in the tank is below 30ppm then the indicating solution will be blue. If it's over 30ppm it will be yellow, and if it is close to 30ppm the solution will be a shade of green. 

At too great a concentration, carbon dioxide can cause problems. As carbon dioxide levels increase, so too do levels of carbonic acid, and these can cause the pH of the tank to drop dangerously low. This is especially a problem if CO2 is dosed overnight, as it combines with the respiration of plants (at night, plants also produce some carbon dioxide) to cause a sudden drop in pH.  There's also an issue in that excess carbon dioxide is toxic to fishes, resulting in gasping, dashing, jumping and eventual death.

As well as providing a source of carbon for plants, carbon dioxide injection also indirectly helps to limit the growth of nuisance algae. When plants do not have enough CO2, they will lose out to algae that will use what little is available to outgrow them. When carbon dioxide is high, the plants can utilise all of the nutrients available to them and outgrow the algae. 

Note that not all plants require carbon dioxide supplementation to thrive. Many popular aquarium species like Anubias, aquatic ferns and the many mosses will happily grow in a tank without any CO2 injection, although they will definitely grow faster and more healthily if it is available.