Tiny crustaceans such as copepods and amphipods are an important part of the diets of many marine fish and invertebrates. They are essential for some fish such as mandarin fish, and many other species enjoy them as a treat. If you own a marine aquarium, the cost of regularly buying live copepods to feed your animals adds up quickly. For this reason, many hobbyists raise their own live food. The copepod and amphipod species regularly sold as live food are not difficult to breed yourself.
Fill one or more buckets with dechlorinated water made up to seawater strength. Mix the salt mix with the water to make seawater with a salinity of between 1.018 and 1.021. Add water to the bucket and stir in the salt mix manually, testing with a refractometer or hydrometer to ensure you get the right strength.
Position the buckets in a warm room with a little natural daylight but not direct sunlight. These creatures don't need light themselves, but ambient light keeps their food supply alive. You do not need a heater if the buckets are in a centrally-heated room.
Run a hose from the air pump into the buckets. You can often run two or more hoses off the same pump, for separate buckets. Adjust the valve on the pump so as the air comes out relatively slowly, at a rate of one or two bubbles a second. These creatures do not need highly aerated water, and too much of a current may slow down their reproduction.
Introduce a starter culture of copepods or amphipods to each bucket. Have one bucket for each species you plan to raise, and keep the two varieties separate.
Add enough phytoplankton to each bucket to turn the water a light green. Scoop a little water into the glass jar to check the colour.
Examine the colour of the water daily with the aid of the jar. Add more phytoplankton as necessary to maintain a light green shade.
Starter cultures of copepods, amphipods and phytoplankton are available from aquarium suppliers. Start harvesting the crustaceans for your fish once the water in a bucket is thick with them -- normally 10 to 14 after you start cultivating them. Several bucket cultures ensures a consistent supply of live food. Amphipods often live close to the substrate and may grow better with a layer of fine gravel at the bottom of the bucket.