What Is a Fish-Only Saltwater System?

Predatory fishes, such as eels, are often chosen for fish-only systems altrendo images / Getty Images

Even though a fish-only or FO system is typically described as a saltwater aquarium that contains just that, only fish, it is not unusual to also have some algae and detritus eating crustaceans and snails present. These animals are only included if they are compatible with the types of fish you are keeping. No live corals and other sessile or motile invertebrates, and what can be considered to be "live" rock are added, but instead, decorative type rocks, natural dried or synthetic pieces of coral, seashells, artificial plants, and other such items are used.

What Fish Are Good For This Type of Aquarium

Most any kind of fish "community" is suitable for this type of set up, but this is one most often chosen by aquarists for keeping aggressive or predatory fishes such as Eels, Scorpions/Lions, Triggers, Groupers, Snappers, Puffers, Hawks, Squirrels, Tangs/Surgeons, and Wrasses, as well as large species Angels.

The Fundamental Elements of an FO Set Up

Aside from the standard pieces of equipment and components that are required for setting up any saltwater aquarium, here are the fundamental elements of a fish-only setup:

  • Sufficient Mechanical Filtration: When it comes to keeping any kind of aquarium clean, having good mechanical filtration is essential, but this is especially important if you decide to keep fish that are messy eaters. Typically canister or hang-on-tank style power filters are ideal for this purpose, and you can step things up by buying one that has complete 3-stage filtration capabilities.
  • Filter Buying Tips: Option 1) "Size up" on the filter. For example, if you have a 30-gallon tank, and the filter you are looking at says it is for a 30-gallon, go up to the next size, such as one rated for 50 gallons. Option 2) "Double up" and run two canisters or power filters of the appropriate size.
  • Good Water Movement: It is important to provide good water circulation throughout the aquarium. This aids in the oxygen gas exchange process, allows the aquarium water to be filtered more thoroughly and helps to avoid problems with dead spots in the substrate. Ideally, this can be achieved with a powerhead or two, or less-so by means of using an air pump with stone(s), or a bubble wand. If you desire to do so, a combination of these two elements can be used. If you are planning to upgrade to a reef tank at a later time, you might want to look at the new wavemakers and install one of those. Some of the new wavemakers offer a terrific number of wave-making patterns and intensities which will work quite well for a Fish Only tank.
  • Lighting: Since you only need lighting to enhance the look of the aquarium and the fishes in it, NO (normal output) daylight, actinic, 50/50, and other standard fluorescent bulbs held in a basic aquarium light strip or hood can be used on a fish-only tank. Many new aquarists who are setting up a Fish only tank plan to graduate, over time, to a FO With Live Rock, or even a full reef tank system and will opt for reef grade lighting to begin with as this will eliminate the need to go to the added expense of upgrading their light at a later time.

Equipment Upgrade Considerations

When starting out with a fish-only aquarium, it is a good idea to design your set up with the thought in mind that you may decide to convert the aquarium into a fish-only with live rock or a reef tank system in the future. By choosing to upgrade your filtration and equipment choices in the beginning, these transitions can be made rather easily and will save you money in the long run by not having to buy new pieces of equipment later on, if, or when you decide to advance to the next level.

Colorful Fish Aquarium
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