Reef aquariums require constant attention to their water chemistry. The amount of dissolved oxygen in water is a critical parameter; too little oxygen can harm or even kill fish and invertebrates. You must learn to recognize the signs of low oxygen in your reef aquarium so you can address this problem if it comes up.
You can test directly for oxygen levels, but you also can see signs of low oxygen in other aspects of water chemistry. If you do spring for an oxygen test kit, you must make sure the oxygen level stays above 5.5 mg/L, though 8 mg/L -- the maximum amount of oxygen that can dissolve in tropical seawater -- is ideal. If you see the pH drop throughout the day, particularly at night when the lights are off, this is a good tip off that the oxygen levels are low. This is because oxygen typically is replaced by carbon dioxide, and carbon dioxide lowers the pH of aquarium water.
Fish will show signs of low oxygen more quickly than invertebrates. Most of the symptoms consist of changes in the way the fish move and breathe. Fish deprived of oxygen typically will flap their gill covers more quickly than normal, and may "gasp" for air at the water's surface. Fish also typically will act more listless than usual. In some cases, fish may develop a blotchy coloration from oxygen deprivation.
Corals, the stars of reef aquariums, will not show signs of oxygen deprivation as readily as fish. Additionally, many corals host symbiotic algae in their tissues that produce oxygen for the coral. Corals that lack this ability have a hard time adapting to captivity, and are seen less frequently in the reef hobby. However, while most reef aquarium corals don't have to worry about low oxygen, the dropping pH associated with rising carbon dioxide levels may make them grow more slowly, since water with an acidic pH makes it harder for them to form their skeletons. For the same reasons, soft corals that usually reproduce quickly may stop budding off when the aquarium doesn't have enough oxygen.
Sources of Low Oxygen
Ideally, the optimal conditions for a reef aquarium would prevent low oxygen. Overcrowding a reef tank can lower the oxygen levels, though the buildup of toxic nitrogen compounds, like ammonia, probably would cause issues long before oxygen got low enough to harm fish or corals. Poor water movement also can promote low oxygen levels, but most coral need strong current. This means that most reef tanks feature strong water movement anyway, preventing this problem.