The Salty Side: Saltwater Aquariums for Beginners

Do You Need Fish In A Frag Tank?

When deciding to run your own frag tank, your main motivation is to grow corals out.
But given that you now have another tank, should you take the opportunity and add a few fish that you like.  Aside from the splash of colour they will add, do you need fish in a frag tank at all?

The addition of any fish in a frag tank is normally based on necessity.  Fish are often added to frag tanks for the following reasons:

  • Algae control

    Much like a regular tank, frag tanks are prone to algae breakouts now and again. Algae eating fish are a common choice and if algae does diminish, as it should with good nutrient control, you can supplement feeding with nori.

  • Pest control

    Unfortunately pests are also often an inescapable fact of reef keeping. Both in our main tanks and in our frag tanks we often control and eliminate some of these pests with the addition of fish.

  • Nutrient Management

    Nutrient control in a frag tank is normally very closely managed. We know that our corals rely and depend on both phosphate and nitrate being present in the system. The addition and fish, and therefore fish food, often helps towards maintaining the required levels.

Of course, there are no hard and fast rules about which fish you decide to add to your frag tank, other than the suitability of the fish for the system (you don’t want a Trigger eating your SPS frags) and the volume of the system.

What is the best fish for algae control?

Although not the prettiest looking fish, they do have character. More importantly, they are excellent fish for the control of algae such as Green Hair Algae not only in a frag tank.  Their characteristic ‘biting’ can be quite comical to watch and also leaves ‘kiss marks’ all over a glass with a bit of film algae covering it.

Being solely a herbivorous fish, it is important to ensure that should their algae supply run out, they are supplied with an alternative food such as nori (which once found they do love). 

Tangs, being herbivores too, are known to be excellent algae eaters.  However all tangs need sufficient space and water volume. They are extremely active fish that should not be couped up in a tiny aquarium of any kind.

In a frag tank a Six Line Wrasse is the go to choice for its natural predatory predilection, however they are not algae eaters.

What is the best fish for pest control?

Normally I would advise against the addition of a Six Line Wrasse as they can become extremely aggressive to fish which may be added after them. However in the case of a frag tank they are super efficient at dealing with unwanted pests (just make sure they go in last).

Six Line Wrasse are brilliant at dispatching worms, small shrimp (including pods), parasitic snails such as Vermatid snails, and Flatworms.

Do you need fish in a frag tank
Photo kindly provided by Brian Gratwicke

Other Wrasse species can be just as efficient, but the Six Line is easier to come by and proven, whereas other Wrasses have the potential to be hit and miss.

How do fish contribute to nutrient management?

In the absence of fish, the only methods of keeping nitrate and phosphate at desirable levels is either to feed your coral regularly (which contributes more PO4), or to dose nitrate and phosphate.

For more information on dosing phosphates, read this article.

For more information on dosing nitrates, read this article

The addition of fish requires that you feed them, and in doing so you are adding nutrients to your frag tank. Fish food contains phosphate, and when eaten creates ammonia once excreted – which eventually becomes nitrate.

For this reason, I even keep a couple of Clowns in my frag tank. As well as my love of Clownfish, they also help in contributing towards much needed phosphate and nitrate my corals require.

Fish and coral cross contamination

There is an argument that the addition of fish in a frag tank could lead to cross contamination.

The concern is, that should the fish carry white spot for example, there is a possibility that the corals within your frag tank may inadvertently become a means of transferring the disease from your infected tank to another.

The above makes the assumption that you will be introducing infected fish into a frag tank unknowingly. To ensure this does not happen, you have two options:

  • Use a QT (See how to set up a quarantine tank here)

    This would eliminate the potential for adding infected fish to your frag tank.

  • Use a trusted LFS

    Purchase your fish from a supplier who has a history of being a trusted for stocking healthy fish.

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