Some good points made by Joe.
More food for fodder ...........
Fish aren't hard wired to assimilate the fatty acids found in beef, anymore than they are hard wired to assimilate large amounts of carbs. These excess lipids get stored in & around the organs, and eventually shorten the fishes lifespan. Can these foodstuffs offer amino acids, and solid growth, yes, no question about that, but that doesn't qualify them as being a good source of food.
Dr. Peter Burgess MSc, Ph.D.,of the Aquarium Advisory Service in England, is not only an experienced aquarium hobbyist, but also a scientist that specializes in the health & disease in fish. He has written over 300 articles and five books on fish health and is a visiting lecturer in Aquarium Sciences and Conservation at Plymouth University, where he works with the University of Plymouth training students in scientific research. Among his other positions, Dr. Burgess is a senior consultant to the Mars FishCare business and regularly runs fish health & husbandry courses for aquarists, fish scientists and vets. He's also a regular contributor & Fish Health consultant for the Practical Fishkeeping Magazine, as well as other magazines devoted to the fish keeping hobby.
Below is an excerpt from an article that Dr. Burgess wrote for Practical Fishkeeping Magazine titled;
To which I will also add, when a fish has to excrete excess amino acids, they must first be deaminated via the liver, a process that not only requires energy (which could be used for growth etc) but over time also places an extra burden on the fishes liver.
Kevin previously stated:
Really, how so Kevin? I know a number of people that have been feeding their various puffers pellets, some of them exclusively, for several years with no ill effects. Quite the opposite, all very healthy fish. This includes both marine & freshwater species. I'm certainly not saying that one can't feed fresh food, such as clams etc to a puffer, but to state that a pellet diet is terrible for them is based on what science exactly? Something that you read on an internet chat forum?
As far as terrestrial based grains, carbs, starches, etc ..........
The term "filler" is a bit of a misnomer. No ingredient used in commercial fish food is used to simply fill a void, they are all added for their nutritional value. With regards to carbs/starch, this only becomes a problem when used in excess, which clearly can be seen in some of the lower quality feeds out there.
I do not know of a single species of fish (marine or freshwater) that have been studied for dietary requirements that has not demonstrated the ability to produce natural enzymes such as amylase that allows them to break down, assimilate, and utilze carbs/starch, including those derived from terrestrial grains. While it is true that most species of fish do not use carbohydrates very efficiently, carbs do in fact help synthesize both lipds & protein.
There is no question that some manufacturers get carried away with these types of ingredients as carbs can be utilized as a cheap source of energy, and protein, but this does not equate to these types of ingredients having zero nutritional matter. As long as their inclusion rate is limited, as in a small amount for binding the food, there is no negative. Would I feed a food that is loaded with starch/carbs, absolutely not, no matter if their origin was aquatic based, or terrestrial based, fresh/frozen, or cooked.
Another common misconception is that raw always trumps cooked, in the case of feeding fish that simply isn't true. Most carbs/starches are more easily assimilated by fish if they are processed (heated) first. Yes some vitamins are lost in the process, but certainly not all, and vitamins & minerals can be made up if one uses quality supplements. With regards to carbs derived from terrestrial sources, cooking those raw ingredients (such as running them through an extruder) also reduces most of the anti-nutriinal matter found in these types of ingredients.
As an example, many people believe that raw peas are a great food source for fish, yet that couldn't be further from the truth. Fresh/canned peas are a very poor source of nutrition for fish.
Peas must be processed first, as they contain anti-nutritional matter, such as tannins, protease inhibitors, saponins, cyanogens, and phytic acid, which when consumed in excess can have a very negative effect on the growth & overall health of fish.
Also, the anti-nutritional factors found in peas can vary greatly from crop to crop & season to season. Something as simple as dry weather, or a cold spell, can push tannin levels up drastically.
The heat from processing will reduce most of this anti nutritional matter, but even then most fish can only digest & assimilate so much terrestrial based plant matter, carnivores/piscivores much less than herbivores. All of this has been well documented in commercial aquaculture for many years.
I find it more than just a bit humorous that someone would state that a company that has millions of dollars invested in this area, and whose food is being fed to fish at some of the most prestigious public aquariums in North America, with people such as Charles Delbeek M.Sc., senior biologist at the Steinhart Aquarium in San Francisco overseeing the feeding, wouldn't really know much about fish nutrition, but a DIY Betty Crocker posting on an internet chat forum, would.