Its entirely possible. Think there are clips on youtube of fish who have been clicker trained (though using a tap on the tank glass in place of a clicker) - this is the method used to train orcas and dolphins etc at places like Sea World, or how the likes of Mary Ray and Richard Curtis train their dogs to 'dance' to music with them.
Fish are capable of learning to do things they wouldnt normally do - look at plecs, bottom feeders for whom surface feeding is SO unnatural their mouths are on the undersides of their heads, they have to swim upside down to do it. Yet many do learn to do this (I had one who did).
If you want to try it, well its easier if you have a single fish in its own tank (which is why the clips you'll see on youtube are of oscars, bettas etc).
Then you use classical conditioning to teach the fish that a certain event means 'you win reward'. So if you do have a tank with just one fish in it, try over the next week or so, coupling a specific, clear tap on the tank glass, say a sharp 'taptap', and then immediatly toss in some favoured food.
You'll know that the fish understands when his behaviour shows he is expecting the food reward that always follows the event marker, the taptap.
Then, you have two choices. For a curious fish like an Oscar you can lure the behaviour with a net handle or a finger, so if you want your fish to swim in a circle or through a hoop, have him follow your finger.
As he does what you want, the second he is actually doing it, not afterwards... taptap and toss in food.
If you have a more skittish and active fish.. wait until he does a specific thing (say, approaches a new item in the tank), taptap and toss in his reward. You can build up to a bigger 'trick', say your fish swimming up and touching a certain item, or through the hoop, by rewarding each step towards that... so week one you reward swimming near it, week two you reward swimming really realy close to it, week three swimming and touching etc.
Thats called free shaping.
Theres loads more to it but it is pretty easy to do (ive trained my dog to shut doors and put things in the bin using these methods - not trained my fish but i reckon if he lived alone, my spiney eel would respond very well to it!).
/www.fish-school.com/
