The Salty Side: Saltwater Aquariums for Beginners

What Is A Frag Tank?

frag tank fish

A frag tank is shallow saltwater aquarium specifically designed to temporarily hold fragmented corals, or frags as they are more commonly known.  They are usually no more than 30 centimetres deep. This limited depth allows the reefer to easily access their fragged corals in the tank.

Although the low depth is pretty much standard for ease of fragging, frag tank sizes can and do  range from relatively small to extremely large. The overall size depends entirely on the requirement of the aquarist and its purpose.  These tanks are used by home aquarists, marine fish and coral retailers, aquaculture suppliers and even by the local coral collectors who culture corals in the sea to ship worldwide.

Most are generally DIY or made-to-measure, and can be manufactured from either glass or acrylic depending on your preference and budget.

how to set up a frag tank

Does A Frag Tank Need A Sump?

A frag tank will more than often have a sump for a number of practical reasons:

  • The addition of a sump increases the total water volume of the aquarium. The larger the water volume, the more stable the parameters will. Stable parameters are essential for the growth and colour of corals.

  •  A sump allows for indirect dosing of corals. This could be trace elementsamino acids and balling salts.

  • A sump will free up space in your frag tank. Instead of having a skimmer, heater and dosing tubes on display, they could all be hidden away within your sump compartment.

  • On some occasions, reef keepers do plumb their frag tanks directly into their main display tanks. This allows them to utilise the sump already in use on the main system.

For information on fragging soft coral, read this article.

For information on fragging LPS coral, read this article.

For information on fragging SPS coral, read this article.

How Large Should The Sump Be?

The sump of a frag tank should be as large as you can make it. The larger the better.

This decision is based on TWO important factors:

  •  STABILITY: As already mentioned above, the larger the overall water volume, the more stable your parameters will be.

    If you have the opportunity to remotely plumb your sump (in the garage or in the basement for instance) then make your sump as large as you can.

  • BACK FILL: Most importantly, make sure that your sump is large enough to accommodate excess water from your frag tank should the power go out.

    Once you lose power, your return pump which has been keeping the water between your sump and frag tank balanced, will of course shut down. This means your frag system will experience back fill.

    Back fill simply means that water will obey the laws of gravity. Water will continue to fall down through the inlet into the sump. It will also fall backwards through the outlet back down into the sump.

    Should you not have sufficient water volume space (or pipework set up correctly) the sump is liable to overflow.

How Do You Plumb A Frag Tank?

A frag tank is plumbed much the same way as a regular saltwater tank is plumbed.

This plumbing guide is intended for use by the home aquarist.

The plumbing consists of the following:

  •  OPTION ONE: Inlet (drain) and Outlet (return) only.

    This option comprises of a 20mm outlet and a 32mm inlet. The inlet includes a ball valve to control the flow of of water from the frag tank to the sump.

  • OPTION TWO: Inlet, Outlet and Emergency Drain.

    This option comprises of a 20mm outlet, a 32mm inlet with ball valve and a 32mm emergency drain.

The choice between having an emergency drain and not having an emergency drain come down to your preference.

Purpose of the emergency drain: Should your inlet become blocked for any reason, such as a snail fining its way down into the plumbing, your return pump will continue to function as normal. This has the potential to continue filling your tank unabated until the it overflows.

However, if you had an emergency drain in place, the water in the frag tank would reach a certain depth and then find its way down the emergency prior to the tank overflowing.

I have an inlet and outlet only. I ensured that my frag tank could hold the maximum water volume of both the sump and the frag tank itself.
My tank holds 130L in normal use, but can hold up to 155L maximum. My sump holds 30L in normal use but should something go wrong, 20L would become trapped in the first compartment due to the height of the baffle. I know that my frag tank will not overflow.

12Pcs Aquarium Fish Tank Divider Plastic Egg Crate Aquarium Filter Bottom Tray Grid Isolation Board Partition Net Pane Separator for Mixed Breeding Light Diffuser,15Pcs Sucker Clips Suction Cups,Black
  • SAFTY MATERIA:Aquarium Fish Tank Filter Bottom Isolation make by safe and environmental protection plastic,won't produce safe effect to aquatic animals box water quality.
  • Size:30cmx15cm/11.8"*5.9"*0.3";Package includes:12 Pcs Fish Tank Filter Bottom Isolate Grid and 15 pcs sucker clip.You can easily divide your fish tank into several compartments with these divider holders for breeding or separation.
  • FLEXIBLE TO USE:each Fish Tank Divider can be snapped together to make a large panel,also can be cut easily with wire cutter to your desired size to fit your fish tank.

Where Do You Drill A Frag Tank?

Whether you have commissioned the manufacture of your frag tank or you are going the DIY route, at some stage you will need to determine where you would like the inlet and outlet holes drilled.

Your choice will be either drill the side of the tank or drill the bottom of the tank.

This takes a little more thought than drilling the bottom in that you will need to plan the exact height of the return and drain.

  • The inlet should be no less than 60mm at its centre from the rim of the frag tank. This will ensure that a third of the drain will be proud of the water surface, and that there should be 40mm between the water surface and the rim of the tank.

  • The outlet will be placed no less than 50mm at its centre from the rim of the tank.

    The top of the return will fall just below the surface of the water.

    Therefore, the outlet will be just below the inlet.

  • If you plan on using an emergency overflow, this can be drilled low on the side. Use an elbow to extend the drain to just beneath the rim of the tank. 

The placement of holes drilled into the bottom of your frag tank is entirely up to you, and what would be most suitable.

When using accompanying bulkheads, you are able to adjust the height of the inlet and outlet pipework, which pass straight through, before securing them at your chosen height at the bulkhead itself.
These heights are easily adjusted prior to the tank being filled, but cannot be adjusted thereafter. If you aim for similar minimum heights as detailed above you will find it works well.

Once again, if you are planning on including an emergency overflow, ensure it is just below the rim of the frag tank.

How To Work Out Your Frag Tank And Sump Volume

It is important to work out the total volume of your system, and to know specifically what your frag tank will hold and what your sump will hold.
Knowing this will enable you to work out what your volume requirement will be to eliminate the potential for an overflowing sump or tank.

Use this handy calculator to plan your build.

What Plumbing Do I Need?

The standard requirement for plumbing most saltwater tanks is as follows:


  • 32mm PVC pipe – ordered to the length you require.
  • 32mm x 1/4″ PVC bulkhead
  • 32mm PVC Double Union Ball Valve
  • 32mm PVC 90° elbow – if required
  • 32mm PVC straight connector – if required
  • 20mm PVC pipe – ordered to the length you require.
  • 20mm x 3/4″ PVC bulkhead
  • 20mm PVC 90° elbow – if required
  • 20mm PVC straight connector
  • A strainer – to prevent anything large and unwanted going down the drain.
  • A duckbill outlet – aids in surface movement
  • A 20mm hose connector – to connect your return pump.
Scroll to Top