FOWLR | Fish Only With Live Rock Aquarium

Choosing Between A FOWLR Or Reef Aquarium

One of the most important decisions the saltwater aquarium hobbyist will have to make very early on is deciding whether to go with a FOWLR aquarium (fish only with live rock) or a full-blown coral reef aquarium.

Ultimately, it all comes down to the kind of livestock you want to keep as well as how much you are willing (or able) to spend on maintaining your saltwater aquarium.

Between the two, the FOWLR aquarium is cheaper to maintain!

Far less intense lighting will be required for the tank inhabitants in a FOWLR tank compared to a full-blown reef aquarium with light-demanding corals. FOWLR tanks present a huge cost savings in terms of electricity and are definitely the way to go if you want to have a beautiful saltwater tank that does not cost a lot of money to maintain.

What’s In A FOWLR Aquarium?

A FOWLR aquarium will contain various compatible fish species against an aquascape of live rock and a deep sandbed of between 3 to 6 inches.

Both the live rock and sandbed which will act as a natural biofilter and be a haven for small crustaceans and burrowing worms that the fish will be able to snack on.

FOWLR aquarium keepers will usually choose to display fish like Angelfish (genera Pomacanthus, Holacanthus, Chaetodontoplus, Pygoplytes and Apolemichthys) and Butterflyfish (genus Chaetodon), showing off their beautiful size and coloration. Many of these fish would make a meal of corals and small invertebrates in a coral reef aquarium!

Generally, higher fish populations are kept in FOWLR aquariums than in coral reef aquariums. Corals require more pristine water conditions and maintaining a small fish population definitely helps in this regard.

With ammonia and nitrite kept at zero, most aquarists will keep the nitrate level in most FOWLR aquariums at around 50ppm — levels that will be harmful to most corals.

Fish Compatibility In A FOWLR Aquarium

Fish compatibility issues are magnified in highly populated FOWLR tanks.

Imagine being put into a tight space with a bunch of people that you wouldn’t normally associate with, or that you don’t really like!

For this reason, always ensure that the fish that you put together in a FOWLR aquarium are compatible as far as size, feeding habits and temperament. Large predatory fish, for example, should never be put together with smaller fish that might end up as a meal!

It is also always a good idea to put some sort of aquarium cover over a FOWLR tank. Fish will inevitably chase each other, and in FOWLR aquariums, the incidence of such territorial behaviour is far greater than in lightly populated coral reef aquariums . Sometimes, the fish being chased will make a mad dash towards the water’s surface to escape, causing it to leap out of the confines of the tank and on to the floor.

More aggressive fish species should generally be introduced together to avoid territorial conflicts.

Cleaner Shrimp In A FOWLR Aquarium

Provided that you do not have any fish that will consume or injure Skunk Cleaner Shrimp (Lysmata amboinensis), having a small crew of these wonderful creatures will add even more interest and color to the FOWLR tank.

Other than performing janitorial duties like eating uneaten food the fish leave behind, Cleaner Shrimp will also set up ‘cleaning stations’. Fish will routinely queue and avail themselves of a good cleaning as these shrimp pick off dead skin cells and parasites from their bodies and gills.

It should be noted however, that another shrimp, the aptly named Boxer Shrimp (Stenopus hispidus)¬†will usually not perform fish cleaning duties in captivity, for reasons unknown — even though that is one of their behaviors in the wild. And neither should they be kept in pairs or in groups as they will fight and kill each other, including the Skunk Cleaner Shrimp!.

FOWLR Filtration

Provided that the tank has been fully cycled and is not overstocked, the live rock and live sand in a FOWLR aquarium will form a natural biofilter, effectively reducing harmful ammonia and nitrite to nitrate and nitrogen gas.

Along with regular water changes, nitrate levels in a FOWLR can be brought down to around 25ppm which is well-tolerated by fish.

An oversized protein skimmer, activated carbon and phosphate remover are also important allies in maintaining a healthy FOWLR aquarium.

FOWLR Aquarium Lighting

FOWLR aquariums need very basic lighting. Because no photosynthetic lifeforms like corals or clams are present, all you will need are a couple of T5 or even low-output T8 tubes that fit the length of your aquarium.

For good color rendition, my personal preference would be one T5 12,000k daylight tube and one 14,000k blue/actinic combination. You could, of course, also go with a basic LED lighting setup if you want the shimmer effect which T5 lights do not provide.

Many aquarists also set up moonlights so they can view the nocturnal behavior of their fish.

FOWLR Aquariums and Copper-Based Medications

Copper-based medications are very efficient at treating fish diseases.

Unfortunately, if used directly in the FOWLR aquarium, they also take a toll on the beneficial bacterial populations in the tank, and will wipe out small organisms living on and in the tiny pores of the live rock.

For this reason, it is always important to quarantine any new fish that you plan on introducing into your FOWLR tank.

Similarly, diseased fish should be removed, quarantined and medicated in a separate hospital tank.

Fish-Only Aquariums

While on the topic of FOWLR aquariums, we should also mention that some aquarists also choose to go with fish-only aquariums for even greater simplicity.

Indeed, before the nitrogen cycle and the biological filtration properties of live rock were fully understood, most early saltwater aquariums were essentially fish-only, with maybe coarse gravel for a sandbed and the uniquitous bleached coral skeletons.

Filtration for such fish-only tanks were largely borrowed from the freshwater aquarium hobby and usually incorporated the use of a power filter or canister filter containing filter wool, activated carbon and ceramic chips to house ammonia and nitrite-consuming aerobic bacteria. But since, there was too little anaerobic bacteria anywhere in the tank to break down nitrate, such fish-only soon became ‘nitrate factories’. To keep nitrate at a tolerable level for the fish, large water changes were the order of the day.

Personally, I think that fish-only aquariums are a thing of the past and have no place in modern saltwater aquarium keeping. When you factor in compulsory, regular water changes and weekly canister filter cleaning, maintaining a fish-only aquarium is not so simple after all.

Fish-Only Aquariums And Copper-Based Medication

Fish-only aquariums allow the tank to be treated with copper-based medications should there be an outbreak of diseases such as ich or marine velvet.

Bear in mind, however, that should you choose to medicate a fish-only tank, such medications will also decimate the populations of beneficial bacteria, leading to the possibility of ammonia or nitrite poisoning.

When medicating a fish-only tank, keeping an eye on any ammonia or nitrite levels along with preventive water changes are necessary to keep the tank from crashing.

Read more about:

Saltwater Aquarium Setup

Saltwater Aquarium Maintenance

Live Rock


Ammonia and Nitrite

Nitrate In The Aquarium

pH and Alkalinity

Aquarium Livestock

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