Freshwater

Most of the fish kept in aquaria are found in freshwater.  The lakes, rivers, streams, and ponds on every continent provide an environment for an endless variety of fish.  However, the aquaria industry is primarily concerned with those fish that are found in tropical waters.  This simply means those waters with an average temperature of 75-80°F.

Brackish

Brackish water occurs naturally where rivers and streams enter the ocean.  This mixing of freshwater and saltwater results in a blend of salinities.  Most fish prefer one or the other, and do not inhabit this area, but a few fish have adapted to take advantage of this unoccupied niche.

Saltwater

Saltwater, or marine, fish live in all the oceans of the world.  As in freshwater, our focus is on those that live in tropical regions with temperatures from 75-80°F.  Additionally we can divide saltwater aquaria into three main subsets.  Fish only tanks are those in which only fish are maintained.  Fish and invertebrate tanks might include crabs, snail, and worms as well as fish.  Reef tanks are characterized by the presence of corals—fish and other invertebrates may or may not be present.

Planted Aquaria

These tanks specialize in the keeping of freshwater plants.  Plants that live underwater are found around the world, and present unique challenges and benefits to freshwater tanks.  The keeping of plants may require increased light or supplemental elements.  One may choose, however to utilize the plants as a sole source of filtration, as opposed to a mechanical filter.

Water Gardens

Water gardens are an increasingly popular way to bring fish keeping hobby outdoors.  Ponds and water gardens provide homes for various plants and fish.  The plants can be tropical and temperate species, but the fish need to be those that can tolerate cool temperatures (below freezing).  As such the fish are limited to goldfish and koi.