As I am a fish keeper, I know that the quality of fish food does matter. I always buy high quality good brands, such as New Life Spectrum, Omega One, Hikari.
New Life spectrum pellets are made of fresh krill mostly, while Omega One flakes are made of whole salmon. Compare to other lesser brands whose fish food are most leftover of fish(usually skin, while good parts were processed for human consumption), the nutritious value of good brands do make a difference.
I can see the fish colors are better after I switched to good brands.
It is also said that fish need more than one type of food in order to have a balanced diet. Some food might be rich in certain nutrients, while lack in some other. That is why I keep several different brands of fish food and rotate them on different days of the week.
Well that is just for fish. I don't know anything about other pets' food.
I wonder, if it is also the same with other pet food? Do dog food and cat food also have this huge of a brand and quality differences? Do they also need more than one type of food to have a balanced diet?
