Fry Incubators

One of the challenges associated with raising fry is balancing feeding with maintenance of good water quality.  On one hand the density of food particles needs to be high enough for very small fry to find food easily and quickly.  On the other hand uneaten food results in a spike in nitrogenous wastes (ammonia, nitrite and nitrate) which will cause harm to the fish.  The trick is to pack the fry into a small space so that small amounts of food can be used to reach the right particle density, but still have an overall water volume high enough for the water quality to be uneffected by uneaten food.  The classic ‘breeder net’ or ‘breeder box’ that hangs in the aquarium and allows water from the tank to flow through its walls accomplishes the goal.  Few of these commercially available products are applicable for all fry.  Some allow too much water to flow.  Others have a mesh or gaps that are too large for very small fry.

I use a fry incubator that I build using a basic aquarium store specimen cup, a small sponge filter, a piece of filter bag and some pvc connections.  I like this unit because it accomplishes the task of isolating even very small fry in a larger aquarium, plus it has the advantage of pushing newly filtered water into the chamber with the fry.  Here is a picture of the finished product in use.

The materials needed to build this incubator are:

  • a specimen cup
  • a small sponge filter with a narrow lift tube (I use the flat, round type with 3/8″ tube that extends from the center of the filter)
  • male 3/4″ and a female 1″ pvc bushings that hit tightly together
  • a 2″ square piece of nylon filter bag material or other porous cloth that will not rot (I do not use screen… the mesh is too large for very small fry)
  • a short piece of airline tubing and an airline connector
  • some filter floss

Tools and other materials needed are:

  • a drill with a 1″ hole bit for the pvc bushing to fit through, a bit for a narrower hole for the sponge filter lift tube (usually 3/8″), and a 1/4″ bit for pilot holes
  • a little super glue
  • utility knife

 Drilling the Holes

The only tricky part to building the incubator is drilling the holes without cracking the plastic cup.  I first drill two pilot holes with a 1/4″ drill bit.  To keep from breakingthe plastic I make sure that the surface I am drilling into is held flat against a piece of wood.  It works best if the wood behind the plastic is solid… not a hole.  One hole is drilled into the bottom corner of the cup opposite the side with the hanger.  Place the hole about 1/2-3/4″ from the corner.  The other hole will be at the top of the cup on either the side wall or the front wall.  Make that hole at least 1″ from the edge and corner of the cup.  The final hole in that spot will be 1″ in diameter.  Change to the larger drill bit and use the pilot hole to guide the large bit in cutting that 1″ hole.  Change to the 3/8″ bit and use the pilot hole in the bottom of the cup to make the hole that will be used for the sponge filter lift tube.  Clean off the melted plastic from around the holes with the utility knife.

Assembly 

Insert the male pvc bushing through the larger, upper hole from the inside of the cup.  Spread the piece of filter bag cloth over the outside end of the male bushing and push the female bushing over it onto the male bushing.  The fit will be very tight.  You may have to use a channel wrench to push the bushings together.  Once they are seated they will be very difficult to get apart and the mesh cloth will be pulled tight across the opening between the bushings.

Remove the lift tube from the sponge filter and fit the bottom of the tube through the smaller hole from inside the cup.  Place a few drops of super glue around the base of the sponge filter where the lift tube will be seated.  Push the sponge filter onto the bottom of the lift tube outside of the incubator and hold it there until the super glue dries enough for it to hold.  Set the incubator aside for a few hours for the glue to completely cure.

Attach the piece of airline to the piece of rigid tubing on the lift tube and put the connector on the end of the airline.  This will make it a lot easier to quickly connect the incubator to an air source.  Push some filter floss into the exit bushing from inside the incubator as a prefilter, otherwise the screen will become clogged.

Use and Cleaning 

The incubator can now be used.  It can be hung inside an established aquarium and provide a small space for fry.  Adjusting the air flow will control the rate a water flow through the filter.  I use some java moss or other free-floating plant in the chamber to provide cover for the fry.  Cleaning the incubator’s screen should happen after a batch of fry are grown up to the point where the incubator is no longer needed.  Discard the filter floss prefilter and hold the bushing to a faucet and run hot water through it.  Reverse the incubator and run hot water through the bushing in the opposite direction.  Rinse and wipe out the chamber, and the incubator is ready to be used again.

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