For a variety of reasons, there are limits to how small or large an animal can be in a particular environment. These involve both physics and chemistry.
Bergmann's rule says animals and subspecies in colder climates are larger than those found in hotter climates. Large size makes it easier to conserve heat.
Gravity affects all land animals and most aquatic animals: there are limits to how light or heavy an animal can be: animals living in water can be heavier.
Atmospheric pressure and air density affect many flying animals: at altitude, less oxygen is available for breathing, and less air is available to push against.
Surface tension affects many animals in unexpected ways: land animals need lungs, not gills, small animals can 'walk' across the surface of calm water.
Mammals and birds are homoiothermic: they maintain their temperature by homeostasis and insulation. Those animals which do not are called poikilothermic.
There are limits to what conditions animals can survive: they need certain levels of moisture, oxygen, temperature and food as a minimum for survival.
The turbidity of water may be measured with a Secchi disc, which is lowered until it just disappears, and then raised until it just appears, and averaging them.