Bacterial Blooms and Cloudy Aquarium Water

Cloudy Water,  maybe you have a Bacterial Bloom

Every aquarist has experienced a bacterial bloom at some point. They are common in new set-ups which are cycling, but can happen at any time. The water goes cloudy, almost like someone has poured a drop of milk into the tank, and no matter how many water changes you do, it doesn’t go away. Sound familiar?

I hope to explain here exactly what a bacterial bloom is, the effect it can have, how to treat it and how to prevent it.

The Nitrogen Cycle

To fully understand about bacterial blooms, knowledge of the Nitrogen Cycle is required. If you are unsure of the Nitrogen Cycle or don’t know what it is, it may be helpful to read the linked topic below first. This is particularly relevant if you have recently set up the tank, as the cloudiness is most likely an indication of other problems.

Now what is a Bacterial Bloom (The Cloudy Water Culprit)?

There are 2 types of bacteria at work in our tanks:

Autotrophic Bacteria – Bacteria capable of synthesizing its own food from inorganic substances, using light or chemical energy. Our beneficial filter bacteria are autotrophs.

Heterotrophic Bacteria – Bacteria that cannot synthesize its own food and is dependent on complex organic substances for nutrition. The heterotrophs in our aquariums mineralise the organic waste (break down the uneaten food, fish waste, dead plant matter etc into ammonia).

Contrary to popular belief, it is commonly the heterotrophs which are seen in our bacterial blooms, not our trusted autotroph nitrifiers.

It is the heterotrophs which are primarily responsible for creating the “bio-film” (slimy residue found on the tank walls and ornaments) which builds up in our aquariums.

The heterotrophs are generally bigger than the autotrophs and therefore don’t attach themselves to surfaces with the same ease. They also reproduce much more quickly. Heterotrophs can reproduce in around 15 – 20 minutes, whereas autotrophs can take up to 24 hours to reproduce.

In new aquarium set ups, the heterotrophs get to work quicker than the autotrophs, causing the ‘cycling bloom’ we so often see. Blooms are almost certainly heterotrophic if they are caused by a buildup of organic waste in the substrate, which most, if not all, are.

Bacterial blooms are common in tanks with apparently no organics present (for example, where all that is in the tank is water and ammonia for a fishless cycle). This is caused by the dechlorination of the water suddenly enabling the water to support bacterial populations. The heterotrophs immediately get to work on the organics in the water itself. The severity of the bloom and even whether a bloom happens at all is dependent upon the level of organics contained in the water supply.

Our autotroph nitrifiers are strictly aerobic (require oxygen), but the heterotrophs can be facultative anaerobic (they can switch between aerobic and anaerobic function depending on their environment). Therefore the heterotrophs in the substrate will be in their anaerobic state and breaking down the organic waste into ammonia, but if they bloom up into the water column, they will switch to their aerobic form and will start to convert the ammonia back to nitrite, although very inefficiently. The heterotrophs are around 1,000,000 times less efficient at ammonia oxidization than our beneficial autotrophs as the heterotrophs are not true nitrifiers.

The Effects of a Bacterial Bloom – Oxygen Deprivation

Most of the bacteria in the aquarium are aerobic as it is an oxygen dominated environment, and these bacteria require lots of oxygen. When the heterotrophic bacteria bloom into the water column and switch to their aerobic state, this is a big drain on the oxygen content of the water. Oxygen deprivation is the only risk to the fish which I am aware of during a bacterial bloom, as the heterotrophs themselves are harmless to fish, so good advice is to increase aeration!

To help you to understand why bacterial blooms occur, overfeeding, dead fish or dead plant matter will cause a rise in the reproduction of the heterotrophs in order to break down the organic waste, they re-produce too quickly to be able to attach themselves to a surface and this causes a bacterial bloom. As the ammonia production increases due to the increased mineralisation, the nitrifiers are slow to catch up (as I said above) and so you see an ammonia spike until the autotrophs reproduce enough to take care of it. Contrary to popular belief, bacterial blooms cause an ammonia spike, not the other way around.

It is unclear whether the autotrophic nitrifiers ever bloom into the water column or if they simply multiply too slowly to cause this effect.

Treatment and Prevention of Bacterial Blooms

A thorough gravel vacuuming will certainly help the situation, as will trying not to overfeed. Also, increase aeration as I noted above. Water changes will probably not clear the cloudiness as when you remove the free-floating heterotrophic bacteria, the others will reproduce more to compensate. Given the reproduction rate of the heterotrophs, it would require a 50% water change every 15 – 20 minutes just to stop the bloom getting worse, and even more if you want to make any progress towards clearing the bloom.

However, water changes won’t exacerbate the situation as it will be heterotrophs (which are producing ammonia) which are removed from the water column via the water change. A water change will remove virtually no nitrifying autotrophic bacteria from the tank at all as 99% of the nitrifiers are housed in the filter, not in the water column. Water changes are not essential in clearing bacterial blooms, as left alone; they will usually dissipate within a matter of days.

Reducing the amount of organic waste in your tank is the ultimate solution to treating a bacterial bloom, and avoiding a buildup of organic waste in the tank is the best way to prevent a bloom. The best way to do this is to maintain regular water changes and substrate vacuuming every time a water change is done.  Dallas Aquarium Experts vacuums the substrate on every aquarium service visit.

As I said above, blooms are common in tanks with apparently no organic waste present, most commonly when only water and ammonia are in the tank for a fishless cycle. In this case, there are few easy ways to remove the organics from the water, and so my best suggestion is to sit it out and wait. Water changes with purified water would help as it would dilute the concentration of organics in the water. Reverse Osmosis water would be ideal in this situation.  This is one reason why Dallas Aquarium Experts always use R/O water for all water changes and new aquarium set-ups.

A bloom in an established tank indicates that there is a problem which has allowed a build-up of organic waste, usually in the substrate. This can be caused by excess dead plant matter, over-feeding which leaves food lying around the tank, or leaving dead fish in the tank. None of these are desirable in an aquarium and a bloom in your established tank will certainly indicate one or more of these causes present in the tank. If you experience a bloom in an established tank, improve your aquarium maintenance.