An overabundance of calcium is extremely unlikely in the wild, but it can occur in the home aquarium, particularly in reef aquariums. In reef aquariums, you must add calcium, since corals absorb calcium from the water. Overshooting -- adding too much calcium -- can have serious consequences for aquarium life and water chemistry.
Calcium, pH and Alkalinity
Many aspects of aquarium water chemistry interact with each other. For example, the addition of calcium increases the pH, alkalinity and hardness of water. Calcium is one of the minerals that determines hardness. Meanwhile, pH represents how acidic or basic a water sample is -- and calcium has basic properties. Alkalinity relates closely to hardness and pH in that it represents the water's ability to resist acid's effects on the pH. Again, calcium raises this value, acting as a buffer. So changing calcium levels directly effects all of these other water parameters.
Normal Water Parameters
To thrive, aquarium fish require water like that of the waters of their natural range. Freshwater parameters vary more widely than those of saltwater. For example, different species of freshwater fish can survive in water with pH anywhere from 6.0 to 8.5. For most freshwater aquariums, you can ignore alkalinity and hardness as long as you measure pH. Except in the case of particularly specialized species, pH gives you a clear-enough picture of the water chemistry. However, in saltwater -- particularly reef -- aquariums, you need to monitor pH, alkalinity and calcium ion levels. You want to shoot for pH between 8.2 and 8.4, alkalinity of 2.3 meg/L or 8dKH, and a calcium ion level of 380 to 450 mg/L.
Effects of Too Much Calcium
The effects of too much calcium vary somewhat. Many freshwater species can survive outside their ideal range, and even thrive. Sudden changes to water chemistry damage fish more than keeping fish outside of their ideal range. However, extreme values and sudden changes can stress and kill aquarium fish. In the saltwater side of the hobby, fish and invertebrates are more particular; they stress more easily outside of their ideal conditions. Overshooting the calcium changes all of the other values, and stresses fish and invertebrates. Stress manifests differently in different organisms, ranging from lethargy to erratic swimming or growing more slowly than usual. In extreme cases, too much calcium can kill aquarium organisms.
Effects of Too Little Calcium
In reef aquariums, it is more likely that you will come across a situation caused by too little calcium than by too much, since many invertebrates absorb calcium from the water. In coral and other hard-shelled invertebrates, a lack of calcium will manifest as slow growth or as poor development of their shells and exoskeletons. It may manifest as slower growth or reproduction in species that reproduce quickly in aquariums. Additionally, a drop in calcium means a drop in hardness, pH and alkalinity. These changes can happen fast enough to shock and kill aquarium organisms.