My Aquarium in 6 hours or 2 years
you can do this in either of two ways that i know which work 100%.
1. play the game for about 2 and a half years, checking in on your fish week to week while breeding and feeding each tank (include using plant food)
the tanks i used were pretty different but if you use six large ones you might breed fish faster.
a larger tank equals more space to breed many varieties simultaneously, thus increasing your fish breeding turnover.
you are gonna have to breed and feed fish but don't worry its fairly easy and you get new fish each week.
start with the smaller fish on page 1 and make your way through each page till all the small fish are bred. repeat this process with the
medium and large fish. also remember to exit the game and save but not just return to the wii menu.
2. Time Cheat - this basically consists of saving your tanks after being fed/bred, returning to the wii menu selecting wii options
-> calendar and setting the date ahead 1 week. after doing this you should return to your game to find new fish!!!
feed your fish and plants in all tanks then save/exit, return to the wii menu and set the date ahead another week... repeat.
do this about 100 times give or take and you should have 49 fish and 1 object (small/large) this is equal to about 2 years.
as far as your save data goes, don't worry you can merely save the present date and all your fish along with the objects will be intact.
as far as i can tell from reading various threads around the web, there could possibly be even more fish or items to unlock.