This actually is an interesting aspect of the scenario. For all intents and purposes the fish in question could be expected to behave in a certain way and it simply did not. In the same way that for all intents and purposes a 2kW jug might be expected to boil 1litre of water from 10 to 100 degrees in a fixed amount of time but it only works 80% efficient. Both are cases where an item could be considered faulty. A very GREY area indeed but interesting from a legal point.
Far more importantly though, if i were a store owner who had received thousands of dollars from a persons' custom over a few years, and upon whose generosity for help i had imposed, the very LEAST i would expect to do would be to offer a store credit (especially given that there would have been profit in the initial purchase in any case). A little common sense to retain the continued support of a valued customer - damn right JoeBloggs, it is the least the LFS could have done.
Sure a store may say "no returns", but this must be across the board and with enough flexibility that common sense can take a part in the equation. It sounds like the "newbie" was being drawn in with the offer of bringing something back but the store felt that you were already hooked so they could ignore passing on a favour !!!
Every right to feel service is poor ! Its a shame that one member of staff (the OWNER apparently in this case - which makes it even worse) diminishes the efforts made by others.