The single most common mistake the new fish hobbyist makes is to start a small aquarium. The smaller the volume of water the more difficult it is to maintain a balanced aquarium and the novice fish hobbyist often walks a tightrope trying to control the situation.

In the long run, if one sets up as larger tank as possible, the extra money spent on the initial set-up will be more than compensated for by the avoidance of catastrophes of a small tank. This will become apparent as we discuss the environmental requirements of freshwater tropical fish further.

LETS LOOK AT TEMPERATURE
Fish are ectotherms, hence they mirror the temperature of the surrounding water and each species has its own preferred range. Ideally, the temperature should be kept constant and for a community tank the ideal range is 23 - 26 degrees Celsius.

There are various means of maintaining a stable temperature but the most common method is the combined heater and thermostat. If this method of heating is chosen then a rule of thumb is to provide one litre of water to each watt of power. If too small a heater/thermostat is chosen then a stable water temperature can't be maintained. If too large a heater is used then there is a risk, if the thermostat fails, of cooking your fish. For large tanks a number of heaters should be incorporated into the set-up.

A more sophisticated means of controlling temperature is using a system of substrate heating. Silicon cable is laid out on the floor of the aquarium and gravel placed over it. The advantages of this system over the glass rod heater/thermostat are that it provides a more even distribution of heat, warms the roots of the plants, needs a lower wattage per litre of water to maintain a stable temperature, ie 1/2 watt/litre, and because warm water rises there is a continuous movement of water. The disadvantage, however, is the cost.

HIGH WATER TEMPERATURE (HYPERTHERMIA)
If fish are exposed to a rise in water temperature their metabolic rate increases. Fish swim rapidly, breathe more quickly and are,overall, hyperactive in behaviour. This increased metabolic rate results in a greater oxygen demand but a rise in water temperature causes a fall in oxygen solubility. Hence overheating manifests as an oxygen deficiency. If pollutants are in the water the increased respiration rate would result in greater toxicity to fish. For instance, the percentage of free ammonia rises with increased temperature.

High water temperatures can also lead to increased multiplication of bacterial pathogens and a compromised immune response in the fish, making them very susceptible to disease. If high water temperatures are not corrected, blood vessels in the fins, skin and under the skin swell up and burst and can be seen as streaks of blood. Fish die soon after. There exists a balance between the fish and the water they swim in, termed an osmotic balance. High temperatures upset this balance and this osmotic stress is a major cause of death. Correction of high water temperatures should be done gradually. Adding salt to the water improves osmoregulation and therefore increases the survival rates of these fish.

LOW WATER TEMPERATURE (HYPOTHERMIA)
Fish tolerate a temperature drop better than a temperature increase. Fish exposed to low temperatures are listless, pale and have clamped fins. Abnormal swimming behaviour is seen. The lethal factor in hypothermia is osmoregulatory collapse. Adding salt to the aquarium may help fish over temperature shock.

A sudden fluctuation in temperature, even within a species temperature tolerance range, can be directly fatal or induce stress-causing immune suppression leading to a disease outbreak.

Remember, when introducing new fish to your aquarium, allow time for temperature equilibration.

Information contributed by:
Dr Harry Pisk
Of the North Ryde Veterinary Hospital (02) 9878 4142 or (02) 9888 1833