Just as fish have adapted to local water chemistry, they are used to a particular temperature. Sudden changes in temperature, primarily a rapid drop in temperature, can be very harmful if not lethal. Most species will be able to function for a while at temperatures somewhat separate from their normal range, although some species could reach metabolic failure.
Fish could become sluggish if the temperature is too low, or have trouble breathing if it is too high. Increased temperature results in increased metabolic rate and therefore the oxygen requirement is higher. But there is a concomitant decrease in oxygen content of water as it heats, so the gills have to work harder on both counts. Oxygen necessities themselves vary from specifies to species, yet again it is a function of the natural environment. The more turbulent the water the more quickly it can absorb oxygen from the environment and the higher its oxygen content is going to be. There are two reasons for this. First, oxygen is absorbed at the edge between water and air at its surface, which then upturns in area when in motion. Second, water movement speeds up passage of oxygen that would spread from the surface only very slowly by diffusion without it.
Large bodies of water such as seas or large lakes have massive water movements such as tides and storms that create high oxygen concentrations. Resident fish that are in these areas adapt to these levels and could die by gradual suffocation if deprived of them. Some species from rivers that are fast flowing could also be affected in this way. On the other hand there are bodies of water that have very little oxygen, yet fish survive there because they have evolved to supplement their air supply from the atmosphere directly.
It is important to make sure that acceptable levels of oxygen are maintained in the tank, and use artificial aeration if required. The return form the filter may supply the required turbulence, or you can use an air pump and aeration device. In most cases the increase of oxygen results from the turbulence created as these break the surface and not from the rising stream of bubbles. Aeration and filtration can also circulate oxygen around the tank. You should however be aware that when you turbulence in tanks housing that is not necessary fish from still water may not be able to swim well.
To finish things off in this section we have added a few more dos and don’ts. First, always follow the manufacturer’s instructions when you use chemical and equipment. You should never guess amounts when adding chemicals. You must measure precisely and test the result each time. Do not however take test results as gospel if they don’t make sense as test kits can often have a shelf life. If a result is dramatically different from what seems sensible, test again.