Do you have a simple setup with just a few corals and concerned that your water doesn’t have enough calcium, alkalinity, etc. to maintain coral growth and health? If your coral load is low, or if the population consists of primarily soft corals, then regular water changes with a quality salt should provide your aquarium inhabitants with everything they need. You may have to dose a specific additive to boost a particular water parameter, but high dollar equipment like calcium reactors are definitely not necessary. As your aquarium grows, however, you may need to start supplementing with a two part additive or several different chemicals to elevate certain parameters.