Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Anaerobic bacteria

Sargassum anglerfish, Histrio histrio
Sargassum anglerfish, Histrio histrio


Anaerobic bacteria can play a very useful role in the aquarium but they need very careful handling because things can go very wrong very quickly if the system is set up wrong.

Anaerobic means that there is no free oxygen. Anaerobic bacteria by definition will only grow and thrive where there is no free oxygen.

The bacteria

Aeromonas Vibrio spp are the dominant species of bacteria responsible for denitrification in the aquarium. There are a few essentials if Anaerobic bacteria are to thrive in an aquarium.

  • The absence of free oxygen.
  • A nutrient source (organic carbon) usually in the form of alcohol. Or sulphur can be used as a nutrient but sulphur systems are less easy to control.
  • Nitrate.
  • A large surface area for bacteria to colonise.

If one of these is missing then anaerobic bacteria won't grow in sufficient numbers to make any difference. It is a popular misconception that the anaerobic bacteria use nitrate as a nutrient, they don't. Nitrate is used for a source of oxygen.

Nitrate (NO3) > Bacteria use the oxygen > Leaving nitrogen (N2)

The left over nitrogen simply bubbles of to atmosphere leaving nothing behind.

Organic carbon

Organic carbon is the carbon that is bound in an organic compound, this carbon is fully accessible to bacteria unlike inorganic carbon found in ores, carbon oxides such as carbon monoxide, carbonates and carbide.

Organic carbon of interest to the aquarist is found in sugars and alcohol.

If ethanol is used the dose rate is 1 drop per 10gall (45 litres) of aquarium water daily.

If the ethanol is sourced from vodka then add 5 drops of vodka per 20gall (90 litres) of aquarium water.

Different systems

Marine aquarium where the biological filtration takes place in live rock. Deep in the core of the live rock free oxygen is limited simply due to the lack of flow through to the core. The rock itself is an ideal place for the bacterial culture to grow,  Nitrate will be present even if only at low levels so the only thing missing is the nutrient. Before adding any nutrient it is highly recommended that this is done in conjunction with a skimmer.

If ethanol is used the dose rate is 1 drop per 10gall (45 litres) of aquarium water daily.

If the ethanol is sourced from vodka then add 5 drops of vodka per 20gall (90 litres) of aquarium water.

Fresh water aquariums will require a nitrate filter.

The nitrate filter

Sargassum anglerfish, Histrio histrio

The simple nitrate filter pictured above can be very efficient at removing nitrates from a fresh water aquarium. But it is important that you understand what is going on in the filter or it will likely end in disaster.

You will need a normal external power filter and an empty canister with no pump or electrics, some makers do sell these but an old broken power filter with the impellor removed will also work.

Water enters the power filter as normal but instead of returning to the aquarium once filtered, SOME of the water is diverted to the canister. The two valves are critical for controlling the flow rate through the nitrate filter because the correct flow rate will be the difference between success and failure.

Taking water which has been through a power filter containing some biological media is ideal because that water will already be very low in oxygen. The flow rate through the nitrate filter should be very slow in order to make sure that as little free oxygen gets in to the filter as possible. The exact rate will have to be determined by careful experimentation.

If the flow rate is to fast the anaerobic bacteria may not have chance to use all the oxygen from the nitrate (NO3) and that could mean instead of nitrogen gas being left over it could be nitrite (NO2) that is left because some of the oxygen hasn't been used.

If the flow is to slow then some organic fragments which pass through the power filter could begin to rot in anaerobic conditions and hydrogen sulphide could be produced which would possibly pollute the water in the aquarium.

Things can go wrong

  • Adding a nutrient to the aquarium can cause a bacterial bloom.
  • A serious bloom the oxygen level will drop to potentially dangers levels
  • Opportunistic bacteria could cause infections in fish.
  • The reaction could be incomplete and nitrate could be turned in to nitrite.



Anaerobic: requiring the absence of free oxygen.

Organic carbon: Total organic carbon (TOC) is the amount of carbon bound in an organic compound