heres a question we can ponder over

[Piscineidiot]

In my experience, the answer is both yes and no (at least in the wild). Some predators, particularly the more highly aggressive, and pelagic variety will reject a single small prey item and chase down a large one with impunity. More purposeful foragers seem just as likely to eat a small prey item in their path as reject it. However, if the small prey is present in abundance, predators often focus on getting as much of it into themselves as possible while ignoring larger prey items that are less abundant.

I think what really matters is what the animal is looking for. Small prey often goes unnoticed unless it's in abundance, thus providing an easy meal or nine. A large prey item is much less likely to go unnoticed due to its size, and if it looks like an easy meal (i.e. if it's crippled), will be readily accepted more often than not.

Having said all that, aquarium fish are so acclimated to captivity and conditioned, that they more often than not do not exhibit wild behaviour. So really, it depends on whether or not you want to talk about wild or captive fish. Bottom line seems to be that fish will take the easy meal wherever possible (although there are certain stimuli that attract violent and energetic responses that probably cost more energy than the meal would be worth).

 

[beblondie]

the answer is yes, predators select prey size carefully. There's something called optimal foraging strategy that means animals will get food in the most profitable way, i.e., the most energy gained from prey compared to the most energy used up getting that prey. This applies equally to herbivores as well as carnivores.
Work done several decades ago on sunfish demonstrated that they will swim a small distance to catch small daphnia but a longer distance to catch bigger daphnia. Small daphnia at a far distance will be ignored if small daphnia closer by are present. Since small daphnia provide little energy, the sunfish will only expend a small amount of energy catching them. Bigger daphnia return more energy, so it's worthwhile swimming more to catch them. If less food is about, the sunfish becomes less picky, swimming further to take smaller prey. When food is abundant, the sunfish is more picky, and will swim shorter distances for smaller prey.
This is a classic piece of work and should be mentioned in most animal behaviour test books. Optimal foraging has been demonstrated again and again, so even though still a theory, it does appear that animals largely work in an optimal way. My own BSc dissertation was involved with demonstrating optimal foraging by starfish, animals that don't really have any kind of brain, yet still seem to select prey types depending on which is more profitable.
I'd caution against using optimal foraging theory as an excuse to mix small fish with big predators though. While I've seen it done with, for example, piranhas in big tanks with guppies, it isn't 100% reliable. In an aquarium fish will adopt learned behaviours, connecting the presence of humans with the appearance of food. That's probably going to override their optimal foraging programming.
The above is compliments of Neale Monks some of you might recognize the name from TFH articles
and books he's written

 

[channarox]

very good info here.
i myself would like to try this...
but it usually only works for LARGER fish...
and due to the diversity of species in my tank,i dont think itll work all that well.