Even in the sea, there are mass deaths when conditions become unsuitable for the survival of urchins and other sedentary creatures that are unable to move quickly enough to a more favourable area. I have also seen thousands fish of several different species spend over hour frantically jumping out of the sea when the water the water became unsuitable for their survival. Usually, they can detect minor changes in water quality and will simply swim elsewhere. It is no good reaching for the medicine bottle and dosing the tank like a quack doctor from the nineteenth century. The best remedy is to remove the ailing fish to a favourable environment.
All life can only survive within a temperature range.
Because water has a higher specific heat than air, it will heat up and cool down much more slowly. Consequently, aquatic creatures will inhabit a much narrower temperature range than terrestrial animals. To keep the fish in the temperatures in which it is naturally found. To avoid any sudden fluctuations of water temperature, or introducing any fish into temperatures of more than 2 o C difference. The aquarist keeping British marine creatures usually has the find some method of cooling the water during the summer 1 .