Have you tried fairy shrimp? They are very cheap, and also reproduce very fast. Their egg will (mostly) not hatch unless completely dried, and re-wet. This means they wont populate the fish tanks. There are some rare exceptions where they do lay "wet eggs" but its not enough to compete with hungry fish. I can get 1000 eggs for less than $10 online. Once your first colony takes off, in about 30 days you can harvest more than 1000 eggs. They sink to the bottom, so just drain the container and swish out the bottom to get the eggs. Dry, repeat.

Its been feeding my minnow for less than a year now, with no horrible issues that I can spot.