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What is a "bloom"?[edit]

A bloom in the contexts of the aquarium hobby, usually refers to either a algae bloom or a bacteria bloom. It is usually characterised within an aquarium as "cloudy water".

Algae bloom[edit]

An algal bloom is a relatively rapid increase in the population of (usually) phytoplankton algae in an aquatic system. Due to an excess of ammonia or another nutrient the levels of a one particular species of algae multiplies and becomes very visible.

Typically this is usually seen in Green water algae. So called as the algae is suspended in the water column and makes the water turn green [1].

Bacteria bloom[edit]

This usually refers to the event when a tank is newly set up and the water turns a cloudy white colour. This is due to high levels of organic material coming off the substrate, ornaments, overdosing of water conditioner, etc. and causing heterotrophic bacteria to briefly multiply in the water column [2].

Within 3–5 days this bacteria bloom clears naturally and is relatively harmless (it may cause a shortage in oxygen so you should aerate the water). Beginners often think something is wrong and attempt to clear the 'problem' by performing water changes. All this does is remove the bacteria and delays the completion of the 'Nitrogen Cycle'.

Other causes[edit]

  • Cleaning too heavily the sponge or media holding the nitifying bacteria may cause a short re-occurrence of a bacteria bloom.
  • Cleaning too much of the substrate at one time may cause a short bacteria bloom. Check ammonia or nitrite levels!
  • Adding a wound treatment like Melafix can occasionally cause a bloom.


  1. Wikipedia
  2. Dr Tim’s Aquatic Library Contribution - The First 30 days by Timothy A. Hovanec, Ph.D. (PDF file)