Autotrophic v Heterotrophic Bacteria in the Aquarium

Almost every bacterial cycling product on the market contains one of two groups: heterotrophic bacteria or nitrifying autotrophic bacteria. Rarely both. They deal with ammonia in completely different ways, and the difference explains why heterotrophic products produce such wildly inconsistent results, dropping ammonia dramatically in one aquarium and doing nothing at all in another. AquaLabs Nitro Cycle contains both, and this article explains why that matters.

Two ways to deal with ammonia

Both groups can lower an ammonia reading. They do it for entirely different reasons.

Heterotrophic bacteria feed on organic matter: uneaten food, fish waste, decaying plant material. That organic carbon is both their energy source and their building material. As they grow, they take up ammonia from the water and build it into their own cells as a nitrogen source. The ammonia goes down, but only because it has been absorbed into bacterial biomass.

Nitrifying autotrophic bacteria get their energy from ammonia itself. They oxidise ammonia to nitrite, and nitrite to nitrate, and that reaction is what powers them. They take their carbon from the carbonates dissolved in your water rather than from organic matter, which is why they have no interest in fish waste at all.

The distinction is worth holding onto: one group eats around the ammonia, the other eats the ammonia.

Heterotrophs are limited by carbon, not by ammonia

The defining feature of heterotrophs is speed. Under favourable conditions they double in roughly 20 to 30 minutes. Give them organic carbon and they multiply explosively.

But that first word is doing a lot of work. Heterotrophs only take up ammonia in proportion to the organic carbon they are consuming, because they are building cells and a cell needs far more carbon than nitrogen. Ammonia is the minor ingredient. Carbon is what sets the rate.

This has a consequence that surprises people: the amount of ammonia in the water has almost no bearing on how much ammonia a heterotrophic product removes. What matters is how much organic carbon is available for it to eat. Plenty of carbon means rapid growth and a sharp fall in ammonia. Little or no carbon means little or no growth, and the ammonia reading barely moves.

What counts as organic carbon

Organic carbon means carbon that was part of something living. Not carbon dioxide, not the carbonates your KH test measures. Those are inorganic, and heterotrophs cannot build cells from them.

Look at the right-hand column and consider a textbook fishless cycle: a brand new aquarium, an inert gravel or sand substrate, filled with dechlorinated tap water and dosed with an ammonium chloride solution to 2 ppm. No fish, no food, no plants, nothing decaying.

There is essentially no organic carbon in that aquarium at all. A heterotrophic product dosed into it has nothing to eat, so it does not grow, and it does not remove any meaningful amount of ammonia. The reading sits exactly where you left it.

Why results vary so much

This is why reports on heterotrophic products are so contradictory. One aquarist sees ammonia collapse in 48 hours. Another dosed the same bottle, watched nothing happen for a week, and concluded the product was dead or fake. Both are describing their tanks accurately.

Aquarium	Organic carbon	What a heterotrophic product appears to do
New tank, inert substrate, dosed ammonia only	Essentially none	Very little or nothing at all
New tank, aquasoil	High and continuous	Ammonia drops substantially and stays down
New tank, fish added, being fed	Low but building	Modest and gradual effect
Established tank, heavy feeding or waste	High	Rapid drop, often with a cloudy bloom

None of this variation applies to nitrifying autotrophs. They are powered by ammonia, so an aquarium with ammonia in it is an aquarium they can work in, whether the substrate is aquasoil or bare glass. That consistency is a large part of why they are the group that actually cycles a tank.

The aquarium holding its breath

Where there is enough organic carbon for a heterotrophic product to work, a second problem appears, and it is the one that catches people out. The aquarium gives every appearance of success right up until it does not.

When the carbon runs out, heterotrophic growth stops. The population dies back, the cells break down, and the nitrogen locked inside them is released straight back into the water as ammonia. Nothing was converted. It was stored, and then handed back.

Three outcomes from the same bottle. With no organic carbon the heterotrophs cannot grow and the ammonia barely moves. With carbon available they drop it quickly, but that is storage rather than conversion, and it returns when the carbon runs out. Only the nitrifying autotrophs take it to zero and hold it there.

A tank cycled with heterotrophs alone is not cycled. It is holding its breath. The ammonia has been taken out of the water and put somewhere temporary, and there is nothing in the aquarium capable of converting it to anything else.

The rebound arrives at the worst possible moment. Someone tests, sees a low ammonia reading, concludes the tank is ready and adds fish. Now there is a real and continuous ammonia load, no biological filter to process it, and a heterotrophic population already dying back because its carbon supply is gone. That is new tank syndrome, arriving a fortnight later than expected and looking inexplicable to the person it happens to.

Heterotrophs are also responsible for the cloudy white bloom you sometimes see in new aquariums. That haze is a heterotrophic population exploding on available organic matter and then collapsing once it is used up. It is another reason a bloom appears in some tanks and never in others.

Why aquasoil hides the problem

Aquasoil is the clearest case of a tank with abundant organic carbon, and it explains something aquarists notice and struggle to account for: a basic heterotrophic product can look genuinely effective in an aquasoil scape and do almost nothing in the same tank set up with gravel or sand.

Aquasoil supplies both the ammonia and, crucially, a continuous source of organic carbon. The heterotrophs are being fed constantly, so the population does not collapse, and ammonia stays suppressed for weeks. The tank looks like it is cycling beautifully.

An inert substrate supplies neither, which is the top row of the table above.

The soil tank is the more dangerous of the two, because the problem is hidden rather than absent. While the heterotrophs are mopping up ammonia, the nitrifying population is receiving very little of the ammonia signal it needs to establish. Then the soil finishes leaching, the carbon supply falls away, the heterotrophs die back, and there is no biological filter underneath to take over. That failure typically surfaces shortly after the tank is stocked.

It is also part of why we recommend a dark start for aquasoil tanks, alongside a product containing real nitrifiers. You want the soil's ammonia to build a permanent biological filter, not to feed a temporary bacterial bloom.

Autotrophs: slow, fussy, permanent

Nitrifying autotrophs behave in almost the opposite way.

They are slow. Nitrosomonas europaea has a reported doubling time of roughly 7 to 24 hours depending on conditions, and Nitrospira around 12 to 32 hours. Set against 20 to 30 minutes for a heterotroph, that is an enormous difference. In the time it takes one nitrifier to become two, a fast heterotroph population can pass through 50 generations.

The reason is energetic. Oxidising ammonia yields very little energy compared with consuming organic matter, so there is simply not much available to build new cells with. Everything about how these bacteria behave follows from that.

They are also particular about conditions in a way heterotrophs are not:

• Oxygen. They are obligate aerobes. Without oxygen they neither multiply nor convert ammonia.
• Temperature. Optimum growth is around 25 to 30 °C, and growth rate falls by roughly half at 18 °C.
• pH. Nitrification slows sharply below 6.5 and can stall almost completely below 6.0.
• Alkalinity. Carbonates are their carbon source, and nitrification is acidifying: oxidising ammonia releases hydrogen ions that consume bicarbonate. Roughly 0.4 dKH is used up for every 1 ppm of ammonia processed, so a single 2 ppm dose costs you close to 0.8 dKH. Run five or six doses through a fishless cycle and you can strip 4 dKH, which is the whole buffer of a moderately soft aquarium. If KH reaches zero, pH will decrease and the cycle will slow down.

Nitrifiers also attach to surfaces. They secrete a slime matrix and form biofilm on filter media, substrate and hardscape. This is why filter media matters, and why the water column itself holds almost no nitrifying capacity.

What you get in return is permanence, and consistency. An established nitrifying colony converts ammonia to nitrate continuously, scaled to the load you feed it, in any aquarium with ammonia in it. It does not collapse when organic carbon runs out because it was never using organic carbon. That is what a cycled aquarium actually is.

Why nitrifiers come in a liquid and not a powder

There is an obvious question here. If nitrifying bacteria live attached to surfaces, why is Nitro Cycle a liquid?

Because the liquid is the delivery vehicle, not the destination. Dosed nitrifiers are free-floating for a short period, then attach to surfaces and begin colonising. The bottle gets them into the tank alive. The filter is where they end up and where they do the work.

That short window in the water column is also why we ask you to switch off UV sterilisers, ozone and protein skimmers, and remove filter socks, for 48 hours after dosing. Those devices strip free-floating organisms out of the water, and for a day or two after dosing that includes the bacteria you have just paid for.

The format also tells you something important about what is in a product.

Bacillus and many other heterotrophs form endospores. A spore is a dormant, armoured survival state that tolerates drying, heat and years on a shelf, then rehydrates and resumes growing. That is exactly what makes a shelf-stable ambient powder possible, and it is why so many bacterial products are sold as powders.

The true nitrifying bacteria form no spores at all. They cannot be dried without dying. To arrive alive they have to be kept in liquid suspension, which is more expensive to produce, heavier to ship and shorter-lived on a shelf.

So the format is a genuine diagnostic before you have read a single word of the label. A shelf-stable powder sold as cycling bacteria is a heterotrophic product, whatever the front of the packet claims. Live nitrifiers come in liquid.

Does CO₂ injection help a tank cycle?

A reasonable question, given autotrophs take their carbon from dissolved inorganic carbon. The answer is no, and injecting CO₂ during cycling is more likely to hurt than help.

The carbon nitrifiers use is mostly bicarbonate, which is what your KH test measures, and there is normally plenty of it. Cycling is almost never carbon-limited for the autotrophs. What CO₂ injection does do reliably is lower pH, and pH is a far bigger lever on nitrification than carbon availability is.

It is worth noting that this is inorganic carbon, so it does nothing for the heterotrophs either. CO₂ injection is not a way to supply the organic carbon discussed above.

Side by side

	Heterotrophic	Nitrifying autotrophic
Energy source	Organic matter	Oxidising ammonia and nitrite
Carbon source	Organic matter	Dissolved carbonates
Growth limited by	Organic carbon available	Ammonia available, and water parameters
Doubling time	20 to 30 minutes	7 to 32 hours
Ammonia handling	Stored in cell mass, released again on death	Converted to nitrite, then nitrate
Consistency between tanks	Highly variable, depends on organic load	Consistent wherever ammonia is present
Where they work	Water column and surfaces	Attached to surfaces, in biofilm
Forms spores	Yes, in many species	No
Can be sold as a powder	Yes	No, requires liquid suspension
Effect on the aquarium	Fast but temporary, when carbon allows	Slow and permanent biological filter

Why Nitro Cycle uses both

Given everything above, it would be easy to conclude that heterotrophs are the problem and a pure nitrifier product would be better. That is not right either, because a new aquarium has two separate problems and the groups solve one each.

Heterotrophs help with the first, in the tanks where there is organic matter for them to work on. They begin drawing ammonia down within hours, which buys a margin of safety across the window when the aquarium is most exposed. They also break down the organic waste that would otherwise decay into still more ammonia, which reduces the load the nitrifiers have to catch up with. In a bare tank on dosed ammonia they contribute little, which is precisely why they cannot be the whole product.

Nitrifying autotrophs solve the second, and they do it in every aquarium regardless of what the substrate is. While the heterotrophs are covering the gap, the nitrifiers are attaching to filter media and building the colony that will run the aquarium for as long as it exists.

Used together, one buys time where it can and the other builds the filter everywhere. Used alone, heterotrophs give you a result that ranges from a reassuring but temporary test reading to no effect whatsoever, and no cycled aquarium in either case.

How to read a bacterial product label

Common misconceptions

"My ammonia dropped in a day, so the tank is cycled." A falling ammonia reading is not the test. The test is ammonia and nitrite both reading zero while an ammonia source is still being supplied, which is why the fishless method keeps dosing ammonia rather than stopping at the first zero.

"The product did nothing, so the bacteria were dead." Sometimes true, but often not. A heterotrophic product in an aquarium with no organic carbon has nothing to grow on, and a live bottle will look identical to a dead one.

"Bacteria live in the water." Your nitrifying population lives on filter media, substrate and hardscape, not in the water column. Liquid bacterial products are liquid because that is how the bacteria survive the bottle, not because that is where they end up. This is also why a water change does not damage an established cycle, while rinsing filter media under the tap does.

"A bacterial bloom means it is working." A bloom usually means there is a large amount of organic matter available, not that nitrification is under way.

The short version

• Heterotrophs feed on organic carbon and take up ammonia as they grow, doubling every 20 to 30 minutes
• Their ammonia removal is limited by organic carbon, not by ammonia, so results vary enormously between aquariums
• In a new tank with an inert substrate and dosed ammonia there is almost no organic carbon, and a heterotrophic product may do nothing at all
• Where carbon is plentiful the ammonia is stored in their cells rather than converted, and returns when the carbon runs out
• Aquasoil supplies organic carbon continuously, which is why heterotroph-only products can look effective in a soil tank and fail in an inert one
• Nitrifying autotrophs get their energy from ammonia itself, doubling every 7 to 32 hours, and convert it permanently to nitrate in any aquarium
• Nitrifiers form no spores, so they cannot be sold as a shelf-stable powder. Live nitrifiers come in liquid
• They live attached to surfaces, so keep the filter full of media and switch off UV, ozone and skimmers for 48 hours after dosing
• CO₂ injection does not help a cycle. Keeping KH above 4 dKH does
• A product with both groups covers the immediate ammonia and organic waste, and builds the permanent filter

If you want to put this into practice, see how to fishless cycle an aquarium, how to fish-in cycle an aquarium if your fish are already in the tank, or how to cycle an aquarium faster for the conditions that control the timeline.