In the Substrate (Sand and Gravel):

Many copepod species, especially harpacticoids, prefer to live in the sand bed or substrate of the aquarium. They burrow into the sand or gravel, feeding on detritus and organic matter. The substrate provides them with a place to hide from predators, particularly during the day.

Reproduction: Copepods often reproduce in the substrate. The sand or gravel offers a stable environment with a constant supply of food (e.g., decaying matter) and is generally safe from disturbance by larger predators. Copepods can lay their eggs in these environments, which will hatch and eventually mature into adults.

On Rocks and Other Surfaces:

Copepods also like to cling to rock surfaces, particularly those covered with algae or biofilm. In a well-established reef tank, biofilm (a thin layer of microorganisms) provides an ideal habitat for copepods.

Reproduction: In areas with healthy algae growth or biofilm, copepods will reproduce by laying eggs on the surfaces. The rocks provide a safe place for their eggs to develop, and once hatched, the copepods can easily feed on the microalgae or detritus on the rocks.

On the Tank Glass:

Copepods can often be found on the tank glass, especially near the water surface, where they feed on plankton or organic particles in the water column. They tend to gather in the corners or edges of the aquarium, where water flow is slower.

Reproduction: While not their primary breeding ground, the tank glass can serve as a place for copepods to find food. In some cases, they may lay eggs on the glass or near areas with slower water flow.

In the Water Column:

Many copepod species are planktonic, meaning they float freely in the water column. Calanoid copepods, for example, are often found swimming through the water, feeding on phytoplankton, algae, and small particles.

Reproduction: The water column may also contain copepod eggs, especially in the early stages of a population's growth. As copepods hatch, the larvae float freely before eventually settling to more stable surfaces or areas where they can find food.

In the Overflow and Filter Areas:

Copepods often thrive in the filter areas, such as the sump or overflow section of an aquarium, especially if the sump has live rock or refugium space with algae. These areas provide a refuge where copepods can avoid predation from larger fish and reproduce in relative safety.

Reproduction: The relatively undisturbed environment in the sump encourages both the survival and reproduction of copepods. They can form colonies in refugiums, where natural planktonic foods are abundant.

Refugium:

If you have a refugium in your saltwater aquarium (a separate tank or compartment designed for growing beneficial organisms like copepods, macroalgae, etc.), this is one of the best places to encourage a healthy copepod population.

Reproduction: Refugiums are ideal breeding grounds for copepods due to their isolation from the main tank, protection from predators, and access to abundant food sources like algae and detritus.

How to Support Copepod Populations in Your Aquarium:

Provide Food: Ensure there is a constant source of phytoplankton or microalgae for copepods to feed on. You can supplement their diet with specially designed copepod food or live phytoplankton.

Avoid Overfeeding: Overfeeding your fish can lead to excess nutrients and waste, which may disrupt the delicate balance needed for copepod health. Copepods require clean water with a steady supply of organic matter to thrive.

Maintain Water Quality: Keep water quality high by regularly testing for ammonia, nitrites, nitrates, and pH. Poor water conditions can harm copepods and lead to their decline.

Ensure Shelter: Provide areas of low flow, such as crevices in live rock, sand beds, and biofilm-covered surfaces, where copepods can hide and reproduce.

Conclusion:

A healthy copepod population in your saltwater aquarium is often evident through visible movement, increased feeding activity from fish and invertebrates, and signs of reproduction. Copepods prefer to hide and reproduce in areas with food sources, such as the substrate, rocks, and refugiums. By maintaining clean water, providing food, and avoiding predation pressures, you can create a thriving environment for copepods, which will benefit your entire marine ecosystem by enhancing biodiversity, supporting biological filtration, and feeding small fish and invertebrates.