Osmoregulation Problems

• Water hardness influences a fish's osmoregulation. Through this process, water continually passes into the body of fresh water fish, while marine fish lose fluids to their environment on an ongoing basis. All fish need to maintain a specific internal fluid concentration and achieve this through osmoregulation. This process requires energy and newly hatched fish placed into water with incorrect hardness will expend far more energy in an attempt to maintain a constant internal body fluid.
  
  

Toxicity Issues

• Water hardness can affect the toxicity of select aquarium medications. In these cases, the action of a normal dose is influenced and fish can become ill. In all cases, hatchling fish are far more delicate than their parents and are easily debilitated. Hatchling fish will not be able to stand a raise in toxicity from any aquarium water conditioner or medication and may not survive the environmental change or treatment.
  
  

Medical Woes

• Water hardness effects pH in a substantial way. A low hardness reading will cause the pH to drop and significant drops in pH will cause the protective mucus coat on fish such as discus to slough off. These fish lose not only their protective mucus barrier, but also experience immune system failure. Respiratory conditions can also occur, whereby insufficient oxygen is absorbed into the bloodstream and the fish suffocates. Hatchling fish are too delicate to easily survive this physical trauma and invariably die.
  
  

pH-Related Stress

• The pH scale is logarithmic and each change represents a tenfold increase or decrease in the acidity or alkalinity of the water. Fish have not evolved to withstand significant changes in pH and become fatally stressed. Hatchling fish will be less able than adults to cope with these shifts. Water hardness influences the pH of aquarium water and the pH will typically crash in the presence of a low hardness level, causing hatchling fish to die.