Interesting and informative discussion! There are quite a few people on here with good knowledge about commercial extruded feeds. I have an interest and some knowledge about commercial dog foods and a lot of the issues are similar.

When it comes to commercial feeds ash and fiber content is usually a good indicator of quality. Ash content is rarely reported on dog food labels unless a company is bragging. A higher ash indicates poorer quality ingredients, usually more bone content. Ash is what's left after the food is burned at a very high temperature and consists of minerals and also heavy metals and toxic materials. Fiber is usually a filler, something undigestible. High amounts of ash and fiber can result in poor growth. You may have to ask the feed company what their levels are if it's not already on the bag.

I noticed that some values in the posts are expressed as crude protein, another as just protein (assuming crude?). Crude protein is different than digestible protein, digestible protein is what the species is able to assimilate, some crude protein isn't very digestible and produces more waste and less nutrition. Quality of protein and how the food is produced can have a great influence on how digestible it is. If the research is there for fish, digestible protein is what you should compare and will be a lower value than crude protein.

I would feed to your specific needs, most things are a give and take, there is no perfect commercial food! If a maintenance diet is what you need then that is what you should probably feed as you will have less waste.

I found this article very useful, don't know if it's been posted before:
Interpreting a Fish Food Package Label
https://edis.ifas.ufl.edu/fa159

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