I feel your pain, and I've been there.. not necessarily with my fish, even though I love them all dearly, but with my dog Shadow that I had for 22 years. When she passed (we had to put her down due to cancer in her hip) I swore I wouldn't do it again. It took a while to get over the pain, but I eventually adopted a stray who showed up at my door in the cold wet rain, and she helps me to remember all the things I loved about Shadow. She keeps Shadow's memory alive and tangible to me.
So, while I understand that it hurts now, and that emotionally you might want out, I'd like to suggest that you take the time to let your emotions settle before you make any decisions.
Sorry for your loss...