A fish tank experiencing a baterial bloom

Cloudy water due to bacterial blooms.

One of the most frequently mentioned issues in a new aquarium is that the water has gone cloudy – as if someone dropped some milk into the tank. Often the owner decides to do a significant water change and the water is crystal clear again, but the next morning the cloud is back! This is a bacterial bloom, caused by rapid growth of harmless bacteria in the water column.

Primarily the bugs which convert ammonia into nitrite are Nitrosomonas – these seem to be the most abundant ones in a normal filter. These bacteria are termed autotrophic

  • meaning they can make their own food from inorganic chemicals – our good bugs eat ammonia and turn it into nitrite. Plants are also autotrophs, they produce food using water and light energy.

These bacteria apparently grow very slowly, which is why it takes such patience and persistence to cycle a tank. Most ammonia munching bacteria take a long time to replicate – 12 to 24 hours for each cell to divide into two.

The type of bacteria usually associated with a bacterial bloom are of a different type – they are heterotrophic.

  • These bacteria require organic matter for food – such as uneaten fish food, poop, or dead plants or a dead fish.

These bacteria usually will not occur in significant numbers is an established tank, unless something is very wrong with the water, such as poor maintenance or a dead fish, but in an un-cycled tank they can proliferate because of organic compounds in the water itself! Like the fish, they ingest food and excrete ammonia.

The heterotrophic bacteria do not stick to the things in the tank very well, and as they replicate every 15 to 20 minutes most of them will end up free floating in the water, causing it to become cloudy. It is almost impossible to get rid of cloudy water with water changes – if  50% of the free floaters are removed in a 50% water change, they will have reproduced back to their original numbers in 15 to 20 minutes!

A bacterial bloom can consume a lot of oxygen from the water – enough to endanger the fish, so it is important to ensure good aeration if we are going to keep the fish and the “good bugs alive” – the good bugs need a lot of oxygen –

  • 1 gram of Ammonia requires nearly 5 grams of Oxygen and 7 grams of Calcium Carbonate – which is why KH (carbonate hardness) reduces as water ages in the tank.


I have seen numerous posts over my relatively few fish keeping years that a bacteria bloom is good (in a cycle) – “the good guys have arrived… (Yayyy)”, but a bloom has no good guys, at least no useful good guys – these are not the bugs that will colonise your filter – those ones reproduce too slowly likely to even be able to bloom into the water.

A bloom causes an Ammonia spike – many think that an Ammonia spike causes the bloom! But most of the bugs in the bloom are not interested in eating ammonia, they are eating the organics in the water, producing ammonia. A small amount of the bugs in the bloom may consume some of that ammonia, but they are very inefficient at it – so the ammonia will rise.

  • The filter bugs are 1 million times better at eating ammonia! As the good filter bugs replicate much slower, it will take time for them to replicate to catch up…

In a fishless cycle the bloom will reduce when the dissolved organics in the tap water have been depleted. A bloom with fish in will cause an ammonia spike as the blooming bacteria produce ammonia, and the ammonia eaters take a long time to replicate to catch up to this new increased production.

In a fish in cycle it is therefore important to watch Ammonia levels, obviously, but especially when there is a bloom, and to try to ensure the tank has as much aeration as you can manage.

If you have a bacterial bloom in a stable cycled tank, then really it is up to you to sort it out – there is most likely too much organic waste in the tank, or you may be having a cycle bump. A thorough substrate cleaning is going to be necessary – thoroughly vacuum the gravel if you have any, or sift through the sand to try to release anything trapped in it before vacuuming above it, and check the tank and filter for dead plants or fish.

This is perhaps especially true if you have an external canister filter – one member at GoldfishResource.org commented that this filter type can seem to work for a long time, seemingly requiring little maintenance, until nitrates go through the roof because of rotting debris trapped within it. I am resolving to check my filter every 3 weeks from now on….

Activated carbon can help remove dissolved organic carbon compounds, but unless you have a lot in the tap water, activated carbon will mask the effect of poor maintenance, but better maintenance would be the ultimate solution.

For more information come and ask a question at GoldfishResource.org