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Purchase a medium-sized shallow tank. Buy fish that stay small to reduce waste in the water. Get cheap fish, invertebrates, corals, and macroalgae. Use corals that do not require a lot of light. Purchase just a few animals that move freely.
Go with a 20 US gal (76 L) tank, which is big enough for plants and animals to thrive in, but not so big that it is a pain to clean. This amount of water actually requires less energy to maintain than smaller sized tanks! Although a tank under 10 US gal (38 L) might seem like it would take less effort, it's actually a lot of work keeping the water at the right chemical levels. The smaller the tank, the more quickly the quality of the water can change due to evaporation and waste. Research all fish you are considering buying and make sure they don’t grow very large. Small fish are just as pretty and interesting as big ones, but they produce a lot less waste. Smaller animals are usually more active for their size and they have the added benefit of requiring less work to care for.  Good examples of small fish for saltwater aquariums include goby, chromis, and certain types of clown fish.  Small fish will have room to swim and stake out little territories comfortably in a 20 US gal (76 L) tank. They are better for a low-maintenance aquarium and for the environment. This is because in most cases they reproduce well in captivity or are plentiful in the wild. They tend not to die on the way out of the pet store and will not take a lot of extra care to keep alive. The reason that cheap fish, invertebrates, and corals are better for the environment is that they are usually bred instead of being taken from the wild. Taking these animals from the wild can cause environmental degradation. Generally this includes soft corals, which are like masses of little anemones without much of a sting. High-powered light consumes more energy than anything else in the aquarium. It also encourages the growth of microalgae and coralline algae, which are difficult to scrape off the sides of the tank.  Some good items to choose include polyps, soft corals, leathers, and mushrooms. More light also produces heat, which can require a chiller to remove from the aquarium and more air conditioning to remove from the house. Make the aquarium colorful with a lot of macroalgae and soft corals and relatively few animals that move freely, such as fish and shrimp. If it doesn't move, it doesn't produce much waste, which will reduce the amount of cleaning you need to do.  However, some hermit crabs and snails are worthwhile for eating microalgae and stirring the sand.  A large amount of macroalgae, live rock, and live sand is a great bio-chemical filter.