You are also assuming that they don't get bored of foods. (Your assertion that hunger will overcome boredom is true of ALL living things, including humans. It isn't exactly evidence that fish don't get bored.) I have observational evidence (which is all that is really being used in this "scientific" discussion anyway) that my fish go after some food with more fervor than others. Specifically, the fish go nuts for defrosted bloodworms much more readily than any other food I feed. To me (and maybe to no one else) that is evidence that they are truly enjoying that particular type of food.
As I've already pointed out fish have the ability to discern one type of algae from another and will eat one and not eat the other. That indicates some level of ability to choose their food. It's not like they are mindless drones who will eat anything organic available to them. They can pick and choose when they are given options. Have you ever given your fish options? Give some bloodworms with some flake (or pellet if you prefer) at the same time and see which one they go for first.
Obviously hungry fish will eat whatever is placed before them, but that doesn't mean that they enjoy it. Prison inmates eat the food they are supplied with as well. That doesn't mean anything except their interest in surviving and getting rid of hunger pains. Your "scientific" discussion is based on your own assumptions of what a fish is capable of conceiving. You don't believe the fish are capable of enjoyment. That's fine. But, don't elevate your assumptions any higher than mine. Just because the fish can't smile or whistle a happy tune, doesn't mean that it is incapable of experiencing enjoyment. I've already cited behaviors that fish will display when given a CHOICE. They choose to "play" in fast moving water or water change movement. They could just hide in the bottom of the tank or hide. But they CHOOSE to be there. That choice happens for a reason and I believe that it is related to their enjoyment of those activities. And if they can choose that, you cannot PROVE that they don't enjoy certain foods and possibly even dislike others. In fact, I have cited observational evidence to SUPPORT that notion (eating one type of algae and not another).
just because a fish knows which food it prefers, does not mean they "enjoy" it. far less that they would get bored with it. cow eat grass all day. i aint ever seen a cow looking for something else to eat, board or not. same goes for horse and apparently, they have similar intelligence to tropical fish. sure a horse will go for the high, value, food. but not because it is bored with grass/hay. but because it knows it has a higher, energy, value.
truth is, there is not the slighted evidence that animals, apart from apes, get bore at all. and less still to say they would get board with food. thing is, through natural selection, animals that get board with their staple. would simply have died out. its not a survivable trait.
animals in zoos and cages, do not get board. they get psychoses, from their poor environment. suffer from lack of contact with others of their species. we see it as a result of lack of entertainment (boredom) but thats a view conceived from an apes point of view and, untimely, irrelevant..
if none apes, got bored with their food. we would have no animals like cows, sheep, pandas all single staple eaters. boredom needs a mind/brain that can wonder off, imagine if you please, only apes have that facility. (remembering to include, possibly, marine mammals).
speaking "scientifically". we have only evidence that precludes boredom, in lower animals. none (possibly as yet) to indicate it.
(Maybe my reading skills aren't that good. 
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