Instructions
Take note of how large your adult tetras appear when you know they are not pregnant. This will help you determine changes in your tetras. More popular neon tetras grow to be about two inches long, while other tetras don't grow past 1 1/2 inches.
Pick out the female tetras. While there are not many differences between the genders, females do tend to have larger, rounder bellies than males. This will be especially evident when they are spawning.
Watch the females daily to check for any changes in their bellies. In neon tetras, you may even be able to see the eggs through the skin.
How to Know When Tetras Are Ready to Lay Eggs
Tetras come in several varieties, including Cardinal, Neon and Glow Light. Because tetras are schooling fish, include at least five tetras in the aquarium, and leave enough swimming space for them -- about a 15-gallon tank size. If you have male and female tetras in an aquarium, you may end up with tetra eggs. While you may not be able to pin down exact dates, there are a few ways to identify females and determine if they will soon be laying eggs.