Total Alkalinity and Total Hardness
Changes in pH can be stressful to the fish, algae and nitrifying bacteria. Balancing alkalinity and hardness reduces this risk.
pH
A small increase in pH can result in high increases in toxic ammonia. Your pH should be measured in the late afternoon when it is highest.
Nitrite and Chloride
High nitrite results in fish respiratory stress and possible mortality. Managing your chloride level can help to eliminate this risk.
Ammonia
High ammonia reduces fish growth (see figure below).