Unfortunately, you can add exactly 0 glofish in a 3-gallon tank. Technically, you can fit 1-3 glofish tetras and danios in a tank that small, but we highly, very highly recommend against it.
Currently, there are 5 glofish species available – tetras, danios, barbs, bettas, and sharks. Assuming it’s a species-only tank, you can add 8-10 glofish tetras, 10-12 glofish danios, 5-6 glofish barbs, 4-6 female glofish bettas, and 0 glofish sharks in a 20-gallon tank.
Glofish tetras, sharks, and barbs can go without food for 1-3 weeks. Glofish tetra bettas can survive for 10-14 days, while glofish danios can last for up to 2 weeks.
Glofish can get anywhere between 2 to 6 inches long, depending on the species. Glofish tetras, danios, and bettas are small fish reaching only around 2-2.5 inches long. Glofish barbs get around 3 inches long, and glofish sharks can grow as big as 6 inches!
You can keep 5-6 glofish tetras, 3 female glofish bettas, and 4-5 glofish danios in a 10-gallon tank. But glofish barbs and glofish sharks aren’t suited for tanks this small.
Glofish barbs live for 4-7 years, while glofish tetras can live up to 10 years. Likewise, glofish danios live for around 5 years, and glofish sharks enjoy a pretty long lifespan of 8 years. And lastly, the latest addition, glofish bettas, live for 3-5 years.