The system starts with a container, usually a 20 gallon aquarium. I made mine out of a 40 gallon plastic trash barrel. This allows a larger amount of water in the sump. The bottom has an under gravel filter. Regular aquarium gravel covers that. An aquarium filter power head pump mounts on the uplift tube of the under gravel filter.
A platform supports the fry containers and a reservoir above the water level. The support must allow water to flow back into the sump area. I used plastic egg crate material normally used for lighting. You can get it at home improvement stores.
The reservoir must be taller than the fry containers. The purpose of the reservoir is to provide a steady water level so that a constant water pressure to the fry containers can be maintained. One or more overflow holes are placed near the top of the reservoir. Drill holes for plastic valves about 1" below the bottom of the over flow hole. A plastic tube brings water from the power head up to the reservoir. There will be a fairly heavy water flow into the reservoir but comparatively little will flow through the valves to the fry containers. Most of the water will exit through the over flow holes back to the sump. That is fine. This action oxygenates the water. I also put filter floss in the reservoir to get extra filtering of the water.
Aquarium plastic air valves go into holes in the reservoir. Standard air tubing directs water from the valves to the fry containers. The valves are adjusted for the desired water flow, usually one drop every second or two.
The fry containers are small plastic dishes like margarine comes in. An over flow hole is punched about 3/4 of the way from the bottom. Tightly folded up filter floss inserted into the overflow hole allows the water to exit but keeps the fry from escaping. I usually add some Java moss to the fry containers to provide cover for the smaller fry. A snail eats any extra food.
If necessary, put an aquarium heater in the sump area to keep the water at the desired temperature. Finally, a lid over the main container helps to prevent evaporation.
Newly hatched fry get a good start in this system. They have the benefit of the water volume in the sump, yet are contained in a small area so that they have easy access to food. Depending on the species, I leave them in the fry system for 2-4 weeks before moving them to a regular aquarium.