This is a summary of what I've read of ornamental fish nutrition.
Reading fish labels can be difficult and confusing. You will not be able to tell from the label exactly how much of the listed ingredients is in the food. You can still tell something though...
Ingredients (in the states at least) need to be listed in order of amount used, but amount can be dry weight or wet weight, and in some circumstances wet weight can include items that are not what consumers will find in the end product. Moreover some ingredients can be split and make it look to the consumer like it's not a significant part of the end product. (E.g., some type of 'corn' derivative could be the 2nd, 3rd and 4th item, and make up 75% of the dry weight content, while 'whole fish' is listed 1st and makes up 10% of the dry weight.)
Protein listed in the product is 'total protein', not digestible protein. Some proteins in the food simply go through the fish and are excreted. Use of brewers yeast is meant to increase protein without using animal protein.
Some fat% is fine. Too much fat is bad. I aim for around 5-6% as that appears to be similar to what fish generally consume.
Some things that should never be in the fish food are meat meal and poultry meal. From what I've read this can be from dead, sick or euthanized animals of any kind. Also, for poultry it can include bird feathers.
If animal grains are used they should not be the first 2 listed ingredients (or all but one of the top 4.) Preferred proteins should be 1st and at least 2 of the first 4 ingredients. First 2 is preferable. Preferred proteins are things which are both 'normal parts of a fish diet' and include beneficial parts of the prey. Whole krill or whole fish are good examples, but for some fish could include other aquatic animals. Some things like fish 'meal', iirc, can include things that are the leftovers from fish like scales, blood, bones, etc. Whole fish would typically mean what it says: all of the fish. Obviously whole fish is better than fish meal which is better than brewers yeast as protein sources.
High ash contents are a negative as it implies things that are undigestable. Lower is better. Below 10% if preferable.
Most fish consume vegetables in some way (whether directly or from stomach contents of the prey) and vegetables are loaded with vitamins. Some form of vegetable matter in the food is good (e.g., dried kelp, spirulina); if not supplements with vitamins is often used.
If anyone wants to correct my summary, have at it.