Plenty of space for them to float around mockingly when they inevitably go belly-up.
You know why people get fish? Because fish die A LOT, and you don't get emotionally attached to a fish. You can't cuddle with a fish. You can't take a fish on a walk. A fish isn't going to alert you when a burglar burgles you, or comfort you when that bastard breaks your heart (YOU KNOW WHO YOU ARE, TOMMY).
So go ahead and get this fish tank. Fill it with all your technicolor hopes and dreams. Then take a sledgehammer to the sides and SMASH IT. Only then will you have even the tiniest inkling what it's like to love a man who doesn't love you back.