How to Build a Fiberglass Frag Tank

Written by dr. samuel helms
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How to Build a Fiberglass Frag Tank
Many different stony corals can be fragged using special tools. (Jupiterimages/ Images)

Coral "frags" are small pieces broken off from the main body of the coral. The frags will slowly grow into larger corals and are a great way of propagating corals for the reef aquarium hobby. A "frag tank" is simply an aquarium to hold the frags in optimum conditions. Usually, these tanks are about six to eight inches deep to allow more light to penetrate the water. The easiest way to build a frag tank from fibreglass is to use a plywood frame and line the inside with fibreglass. The wood will give strength and support to the tank while also costing less than fibreglass.

Skill level:
Moderately Challenging

Things you need

  • Two-by-four beams
  • Plywood
  • Fibreglass fabric sheets
  • Galvanised wood screws
  • Marine epoxy
  • Aquarium-safe silicone

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  1. 1

    Sketch a scale drawing of the frag tank. Careful planning will help you avoid costly mistakes when building the aquarium. Make sure you include areas to attach the frag tank to the filtration system (a sump) and perhaps areas to increase water flow, such as closed loop systems. Good water flow is important for the health of the coral frags.

  2. 2

    Build a frame for the aquarium using two-by-fours. Think of this like a skeleton that will support the weight of the water. Unless the plywood will sit firmly on a flat surface, you should build a skeleton of wood under the bottom of the tank. This will give it extra strength and help prevent warping as well. The sides will need only one two-by-four running their lengths, since they are not that tall. Space the beams on the bottom four to six inches apart. Use galvanised bolts and screws to hold the wood together.

  3. 3

    Cut and add the plywood sheets to the inside of the wood frame. Use galvanised wood screws and countersink them in the plywood. When you have it screwed together, fill the holes of the screws with wood filler.

  4. 4

    Sand the inside of the plywood. It needs to be very smooth for the next steps.

  5. 5

    Coat the inside and outside of the wood with marine epoxy. This epoxy is often used for boats and is safe for the frags when dry and cured. Do this in a well ventilated area or outside because the fumes are toxic.

  6. 6

    Cut your fibreglass fabric and add the pieces, one at a time, to the inside of the plywood. Thoroughly coat the fibreglass sheet with epoxy. There should be no air bubbles or white spots on the sheet, and it should look nearly transparent.

  7. 7

    Coat the inside of the frag tank with four more coats of epoxy, waiting four hours between coats.

  8. 8

    After the epoxy has fully cured, add a bead of aquarium-safe silicone to all corners and seams inside of the frag tank. The epoxy likely filled all the holes, but this step will help ensure there are no leaks.

  9. 9

    Let everything fully cure according to the manufacturer's instructions.

  10. 10

    Slowly fill the tank with water, checking for leaks. Mark where the leaks are and after draining and drying the tank, seal the leaks with epoxy or aquarium-safe silicone.

Tips and warnings

  • Good flow over the frags will help ensure they grow strong skeletons.
  • Wear a respirator if you use the epoxy indoors.

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