To get this trophy you will need to have most of the fish unlocked. Fish unlock as time passes (real time, not game time), so this can be manipulated if you wish. You can do this by either loading up the game, then exiting back to the XMB and putting your consoles date forward a year and loading the game back up, which should also unlock
Or you can just load the game up every few days. I did it this way and it took roughly 6 - 8 weeks before I had the correct fish to get this trophy.
Make sure you choose a large fish tank by cycling through them using L1 or R1 until you get to a rank with "L" after the number (E.G. 5L, 6L, etc.) or you can choose a small (S) or medium (M) tank, go to Aquarium Design, Remake Aquarium, and change your selected aquarium to large. Then add one of each of the following fish:
Blue Grass Guppy
Press O after you add your final fish to return to the Aqua Center and unlock your trophy.
A huge thank you to zantres
for taking the time to help me figure out the best fish to get this with.
Please vote positively if this guide was helpful and feel free to leave a comment if I can improve it in any way.