I used to keep gold fish and sucker fish. In fact, I'm glad this is posted. I'm going to restock. The sucker fish help hoover the algae, and the goldfish eat everything else. The suckers are sub-tropical and cannot take a winter. A pet store is happy to take your sucker fish returns, and will restock you the following spring/summer with new ones. Make sure you return them to a pet store by September, or they will die. Goldfish are pretty hardy. They will be pretty miserable without a cinder block. They need a home/shelter like everybody else. Plus, we have Jack Russells, so the blocks can save one from drowning. My fish grew quite large, and multiplied. When I did empty the trough (which is a quite the chore) I turned the goldfish loose in a stock tank.