The aquaponic system must have enough fish or other aquatic life to produce sufficient nutrients for the plants. This ideal “bio-load” is always in flux and will vary depending on the number and size of plants in the system. For the ECO-Cycle 20 gallon system, the ideal number fish will also vary, but here are some good targets: Small tropical fish (tetras, guppies, platies, rasboras, small gouramis. etc.) – 30 inches of fish Medium tropical fish (peaceful cichlids, catfish, mollies, medium gouramis, etc.) – 20 inches of fish Goldfish – 15 inches of fish These target fish populations are best built up incrementally. Start with just a few hardy fish. After the system has been running a few weeks, add a few more; wait a few weeks and add a couple more, etc., until the target bio-load has been reached.