All fish and plants have adapted to their environment over time and are quite specific about what water temperature they are comfortable in. Maintaining a consistent water temperature helps to keep fish, plants and other occupants healthy and less stressed. Fluctuations are normal, even in nature from rain or outside temperature the levels ebb and flow due to different conditions. So don’t be too worried if there are changes in temperature. Just as long as you are aware of any sudden big changes.
When doing water changes try and keep the water at a similar temperature to the left over water in the aquarium. Letting the water sit over night in a holding tank is usually the easiest way and if too cold a heater can always be turned on to bring the temperature up to the desired level. Another way is to add small amounts of water to the tank over the day so the water adjusts slowly to the temperature of the aquarium. Temperature of larger tanks are easier to maintain as with a larger body of water, the temperature change is more gradual than smaller tanks.
The temperature of an aquarium is usually a few degrees lower than the temperature of the room it is in. Some aquarium shops like to heat their entire premises constantly as this is easier than having to have heaters in every tank. They can easily adjust the the temperature of their tanks by adjusting the room temperature up or down a few degrees.
A simple floating thermometer or one that sticks on the glass is all that is needed to check the temperature periodically – once a day, to make sure all is right in your aquarium.
The aquarium heater is the most commonly used piece of equipment to heat an aquarium. You can also use a substrate heater or chiller to maintain temperature control. Lights on a tank can heat up the aquarium also, so check to make sure they are not adding too much heat to the aquarium, especially in summer.
Koi and Gold fish like cooler temperatures compared with tropical fish and can live a temperatures a lot lower. Best not to keep these fish together with tropical fish as they enjoy a different environment. They can withstand warmer temperatures, as in summer if in a pond the water will become warmer but they prefer it to be in cooler temps.
Plants are sensitive to the temperature of the water but not as much as fish. Still ideal to keep most tropical plants in a warmer tank as if too cold the plants will become more dormant and growth rate will slow significantly. That said there are certain types of plants that are happy in cold water so these are ideal if in colder environments.
If fish are sick or have parasites raising the temperature in the tank is good practice to help fish deal with the problem plus also adding medication as well at the same time to combat the specific disease or infection.
The above is just a few different points regarding temperature that you may already know but is always good to brush up on when owning an aquarium.