FCR is a complex function of metabolic consumption and conversion efficiency of excess consumption. These are variable being temperature dependent. Without highly controlled experiments the truth of claims of conversion efficiency are suspect ... especially when comparing brands.
To be sure. None are identical and yet they are very similar ... meaning ... they have many of the same ingredients.
I would just offer this advice. Get whatever you want but err on the side of higher protein and fat contents. If the product lists ingredients, err on the side of fish sourced protein and fish sourced fat. A 48% fish food will have more than twice the amount of fish meal than a 40% food (assuming fish meal is the primary source of protein in the 48% fish food AND especially if the feed has a high fat content). What is certain, particularly if the fat content is high, a 48% food will source protein primarily from animal proteins which are more digestible for predator fish. The conversion of 48% feeds should be better than 40% feeds generally for predators like BG and LMB ... enough to make it worth the price.
