Fish grow best when kept in their ideal conditions, ie fish that preffer hard alkeline water in hard alkeline water and fish that preffer soft acidic water in water that is soft and acidic. If the chemistry of the water you use for water changes has changed then it could effect fish growth and health.
Other factors that will effect fish growth are the quality of the water (how much nitrogen pollution and other toxins it contains) and the quality of foods, fish raised in good clean water and fed live foods will grow much quicker and larger than those kept in tanks with poor water quality and fed flake and pellet foods.