Lipids (fats) are high-energy nutrients that can be utilized to partially spare (substitute for) protein in aquaculture feeds. Lipids supply about twice the energy as proteins and carbohydrates. Lipids typically comprise about 15% of fish diets, supply essential fatty acids (EFA) and serve as transporters for fat-soluble vitamins.

A recent trend in fish feeds is to use higher levels of lipids in the diet. Although increasing dietary lipids can help reduce the high costs of diets by partially sparing protein in the feed, problems such as excessive fat deposition in the liver can decrease the health and market quality of fish.

From http://pubs.ext.vt.edu/420/420-256/420-256.html

US National Research Council: "Nutrition Requirements of Coldwater Fishes" concludes 10-20% lipids for cold water fishes.

This one is very interesting:
http://www.iffo.net/es/system/files/TB25%20Fish%20Oil%20Salmon.PDF

It goes through a summary of several studies and concludes that optimal lipids varies with species. It makes a difference on source of lipids, especially fish oil (typ menhaden) vs vegetable (corn/soy/etc). Redfish (channel bass) fed a diet whose lipids were mostly from menahaden did best at 7.4-11.2%. For catfish and tilapia, they cite 10% being optimal.

I do recall reading an article a while ago that lipid requirements vary with age as well. We know folks typically run a special feed for fry and young.

So it would seem that "more is better" is not accurate. There would be some upper limits.