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.