Water Temperatures and Fish Health

  The Fish Environment

Water temperatures and fish health

Fish are ectotherms or poikilotherms. These are both terms that describe animals that have a body temperature that is dependent on the environmental temperature. Such animals are commonly, but incorrectly, called ‘cold-blooded’.

Because of this dependence, environmental temperatures have a profound effect on many aspects of aquatic health. Every species has its own minimum and maximum temperature range and the animal’s health is likely to be affected at temperatures outside that range.

Water temperature and oxygen solubility

The solubility of oxygen in water reduces as temperatures increase (see table 1). Conversely, because of an increased metabolic rate the oxygen requirement  increases as water temperatures increase. Clearly, if the total oxygen demand of the system, which includes fish, bacteria and submerged plants, exceeds the dissolved oxygen levels the fish are likely to suffer.  Although in a well-aerated pond or aquaria this would be unusual, it can happen when;

  • water temperatures are very high – especially with high humidity
  • the stocking densities are too high 
  • there is a lot of organic matter in the system because organic matter uses up substantial amounts of dissolved oxygen when it decomposes
  • there are large amounts of submerged plants and algae, because they remove oxygen at night (see photosynthesis)
  • disease treatments such as formalin are used in high temperatures because they remove oxygen from the water
Water temperatures oC
5 10 15 20 25 30

Oxygen saturation in mg/l

12.8 11.3 10.2 9.2 8.2 7.5

Minimum oxygen requirements of fish (mg/l)

9.1 8.8 8.3 7.8 7.4 6.9

Table 1: Dissolved oxygen levels at various temperatures

Adapted from the Interpet Manual of Fish Health


Table 1 shows that as water temperatures increase the gap between the level of dissolved oxygen and the minimum oxygen demand of the fish closes. It is worth noting that under low oxygen conditions (hypoxia), the biggest fish will suffer the most because they have a greater oxygen demand. At high temperatures it is important to keep the system well aerated – especially at night when plant stops.

Water temperature and fish metabolism

We have already mentioned the fact that fish metabolic rate is dependent on water temperature. In winter their metabolism slows right down and speeds up as the water gets warmer in spring. This dependence on water temperature also affects their immune system, wound healing and digestion.

For most cold water fish, the immune response is severely inhibited below 12oC. Even when water temperatures start to rise there is a time lag of a week or so before the fish immune system starts to function effectively. This is one of the reasons that fish are particularly vulnerable during spring, when water temperatures often hover around the 12 -15oC mark for several weeks. Unfortunately fish parasites and opportunistic bacteria tend to become active a lot quicker as temperatures start to rise and often take advantage of sluggish fish.

Water temperatures and disease treatment

Many common disease treatments are temperature dependent both in usage and dosage. For example treatments such as malachite green, formalin, chloramine-T and many proprietary treatments should not be used below 10oC. For treatments such as organophosphates and chloramine-T the recommended dosage increases with higher temperatures.

The dependency of the immune system and wound healing means that any supportive treatments, such as antibiotics and topical treatments, use to assist fish with lesions, bacterial infections and ulcers need to be carried out at temperatures of 15oC or higher if they are to be effective.