One thing that I see everybody getting hung up on is the protein/fat ratio, i.e. more is better, right? I was the same way a year ago, now I know a little bit more. It's a good "guide", but it isn't the only thing that needs to be looked at. You also have to look at what species of fish the food was designed to feed.
For instance, if you have cat food at 30%/15% and you have dog food at 30%/15% and cat food was $10/bag cheaper would you buy cat food for your dog? You may laugh at the comparison, but it's not so far fetched when talking about different species of fish and their food requirements.
Proteins can be sourced from many, many different things. Some are not very easily digestible by fish, others are. There are different trace/micro nutrients and amino acids that are in the different foods. Different fish species need different blends. Different protein sources that may be not very digestible by fish can be made more easily digested by modifying the other ingredients in the food. Also, different fish species respond to different protein/fat sources than others. So, it's not just a matter of what protein/fat levels.
That is why there is food for different species of fish. If there wasn't a need for that, then there could be a maximum of 3 different fish feeds. High protein/high fat/small pellet size for fingerlings, medium protein/medium fat/medium pellet size for middle of the road fish, and low protein/low fat/large pellet size for the largest fish in the pond.
Will the different fish food producers tell you what trace/micro nutrients and amino acids are in the food? I highly doubt it, as that is part of their propriety information that makes their food "theirs" and how well it works.
