Copepods are Nature’s trophic intermediaries. That is, they transfer nutrients and food energy from lower (e.g. plants) to higher (e.g. carnivores) levels in the food chain. This also happens to make them nearly indispensable in aquarium systems, where they (1) consume unwanted material such as nuisance algae and detritus and (2) are eaten by zooplanktivores ranging from fish (anthias, mandarins, etc.) to corals. Their young, quite notably, generally are phytoplanktivores, eating free-living microalgae from the water column during their planktonic larval stages.
As it is comparatively more nutritious, pods prefer algal material to detritus. They mainly consume soft microalgae. This means that they typically do not eat crustose forms such as coralline algae, nor do they eat macroalgae (aside from dead, decaying, softened material). They love green algae and diatoms; these are both available in a benthic form as thin films (for adult harpacticoid copepods) and in a planktonic form as single cells (for larval harpacticoid copepods and all cyclopoid copepods). As such, a diverse community of copepods helps to keep the aquarium water clear and the rocks, sand, and glass free of green or brown films.
Being so beneficial, aquarists generally seek to culture as high of numbers of these critters as possible. Unsurprisingly, their population densities are limited by food availability. This means that if you add pods to remedy a preexisting algae plague or detritus pool, you might observe steady increases during the exponential growth phase, only to see numbers drop as food supplies are gradually depleted.
Sometimes a pod population will decline before the nuisance algae and/or detritus is gone, or worse yet, they never really take off at all. Where losses to heavy mechanical filtration (e.g. filter socks) or
UV sterilization can be discounted, this usually due to the starvation of larvae. Remember, their “babies” need phyto! Those aquarists who dose phyto on a regular basis maintain larger pod populations (can’t have lots of adults without lots of larvae). And, when raised on a nutritionally balanced live phyto product such as OceanMagik, your pods will be more nutritious for the animals that prey on them!
Phytoplankton is essential to maintain a stable copepod population. This is especially important if there is a finicky fish such as a mandarin dragonet or leopard wrasse relying on the copepods for food. A mandarin can eat thousands upon thousands of copepods in one day. Remember, it what they spend most of their time doing. So, keeping your copepod population as stable as possible is incredibly important. Even if you don't have a finicky fish to feed, it is still something to strive for.
The great thing about phytoplankton is that it too will feed your corals. Phytoplankton also competes with nuisance algae in your aquarium by consuming nutrients. So, not only does phytoplankton indirectly keep your aquarium clean by feeding copepods, it also directly keep your aquarium clean. It would be highly beneficial to add phytoplankton to your aquarium even if you weren't concerned about feeding copepods. Phytoplankton and copepods will work together to feed your fish and corals and keep your aquarium clean. They also keep your aquarium more stable in general, making it closer to a real ecosystem!
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