Small Floating Fry Container

I am a big believer in starting out fry in a relatively small container that is part of a larger water supply. The small size means that the fry don’t have to exert much energy finding food. If you put the fry in a larger tank either you have to put a lot more food in it (and risk polluting the water with uneaten food) or make the fry hunt for it. Also you might not want to dedicate a tank if the spawning only produced a few fry.

 The problem is that the water in a small container is more likely to go bad. Some aquarists use small containers but do frequent water changes, which is a lot of extra work. I got around these problems with my continuous flow fry system. The fry are contained in small cups but water from a much larger volume circulates through each fry cup.

That is what I start the majority of my egg laying fry, which are mostly killifish but it also works with other species such as Corydoras. The problem is I sometimes breed species that require different conditions than the continuous flow system which is set for, about 75°F and pretty hard tap water.  Sometimes the fry need warmer or cooler temperatures or maybe very soft water or brackish water.

I came up with a solution that while not is as good as the continuous flow system but better than just small containers.  It floats the containers in the parent’s tank, which will already have the desired water conditions.  There are fry rearing cages available with netting sides. They will have excellent water flow, but any small food like baby brine shrimp, microworms,  or paramecium will just go right though the net, starving the fry.

Start with a small plastic food container. Mine are made from the containers potato salad from the grocery store comes in. They should be fairly deep. A large rim is also helpful. Clean them well but of course don’t use soap which can poison the fish.   Punch a couple of holes about ½” or slightly smaller diameter about halfway down. The exact size is not important.  

Floating Fry Container

Fry raising container in the parent's tank.

Assembled floating fry container.

Next find a sheet of plastic foam.  Cut a size that will extend around the container by about ¾”-1” on all sides. Then cut out the center with a sharp knife or razor blade. The container should slip through the opening to the top, but should have a snug fit.

The final step is to plug the holes with filter floss. Take a small piece and fold it back on itself several times to form a small wad a little larger than the hole. One end will be fuzzy and the other fairly smooth. Take the smoother end and insert it into the hole from the outside. If you just shove the floss in the hole or insert the fuzzy side the fry and/or food might get caught in the floss.  The floss must be snug in the hole so that it does not fall out and allow the fry to get out the holes. The floss will eventually get clogged with detritus so you will need to change it about every 2 weeks.

Just put the container in the tank and hold it down. Water will flow through the holes. Once the water level inside reaches the holes, water will continue to flow in until the foam is level with the water’s surface and the container will float nicely.

There will be little water flow between the tank and the container.  Before you feed the fry, just lift the container out of the water and it will start to drain back into the tank. Once about half of it has drained out, just put it back in the tank and it will refill by itself. It will take less than a half minute to do the draining process. You could put additional holes to make the process even faster. Since you likely feed the fry several times a day, you are doing several 50% water changes a day with very little effort.  The water temperature and chemistry will already be correct with no additional work on your part.

This system costs almost nothing to make and will solve a number of problems in getting your fry off to a great start.


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