GloFish Breeding: Eggs, Rules and How Danios Spawn

GloFish are genetically modified aquarium fish that carry a fluorescent protein gene, and their offspring glow too. GloFish danios do breed in home aquariums, but the GloFish license that comes with the fish prohibits intentional breeding and any sale, trade or giveaway of the offspring.

This guide covers what GloFish are, what their eggs look like, how danios spawn, and what the rules say.

What Are GloFish and Where Do They Come From?

GloFish were developed at the National University of Singapore, where researchers led by Dr. Zhiyuan Gong inserted a fluorescent protein gene into zebra danio embryos. The original goal was a fish that would light up in polluted water. The first fish glowed all the time, so they were instead brought to market as pets. Yorktown Technologies began selling them in the US in 2003, and the brand has been owned by Spectrum Brands since 2017.

The fluorescence is built into the fish’s genes, so GloFish are bred like any other fish. They are not injected or dyed. They are patent-protected, which is why the brand controls their sale.

How Many Species of GloFish Are There Today?

The range has grown over the years. The best-known GloFish are:

• GloFish danios, the original and based on the zebra danio.
• GloFish tetras, a black-skirt-type tetra, which do well with other small peaceful fish such as danios, mollies and platies.
• GloFish barbs, based on tiger barbs, which are fin-nippers and best kept in a group.
• GloFish sharks, based on the rainbow shark. Despite the name, this fish belongs to the carp family (Cyprinidae), not to sharks. It needs a much bigger tank than the others, and it is aggressive toward its own species, so keep just one.
• GloFish bettas, and newer additions including X-ray tetras, bronze corydoras and angelfish, depending on where you shop.

Danios pair well with other small, active fish like guppies, swordtails, barbs and rainbowfish. Most of the others need a tank of their own or careful tank mates, and none should be kept with dwarf cichlids unless the tank is large and well planted.

Where Are GloFish Illegal?

In the US, the FDA has chosen not to regulate GloFish, because ornamental aquarium fish are not food and are not considered a greater risk to the environment than unmodified fish. California banned them from 2003, but lifted the ban in 2015, so they are now sold in all 50 states. GloFish are not authorized in the European Union.

Rules differ by country and change over time. In Australia, for example, live fish imports are tightly controlled. Check your own government’s rules before you buy. No matter where you live, never release GloFish into the wild.

Do GloFish Stay Fluorescent When They Breed?

Yes. The fluorescent gene is inherited, so the offspring of GloFish glow as well. How strongly they glow can vary from fish to fish.

How Often Will GloFish Lay Eggs?

Take the GloFish danio, the most common type. Its spawning behavior is the same as the regular zebra danio fish. Danios are egg scatterers, not livebearers, so the female does not get pregnant and give birth. She drops eggs in the open, and males fertilize them as they fall.

In a well-fed, well-conditioned group, danios can spawn often, in some cases every few days. Danios spawn in groups, and one batch of eggs may be fertilized by more than one male. A female can release 100 or more eggs per spawn.

What Do GloFish Eggs Look Like in a Tank?

Danio eggs are tiny, round and transparent, less than a millimeter across. They are not colored and they do not glow at first. They are scattered loose and may stick to plants or settle between substrate and decor.

Healthy, fertilized eggs stay clear, and you will soon see a small dark dot (the developing embryo) in each one. Eggs that turn cloudy or white are generally dead or unfertilized. Hatching takes about 1 to 3 days depending on temperature. The newly hatched fry are very small, and they feed from their yolk sacs for the first few days. Adult fish will eat the eggs if they get the chance.

Is It Illegal to Breed GloFish?

GloFish are sold under a license that states that “intentional breeding and/or any sale, barter, or trade, of any offspring of GloFish fluorescent ornamental fish is strictly prohibited.” That makes breeding for sale off-limits and breeding on purpose a breach of the license, though accidental spawns do happen when the fish are kept in good conditions.

If your fish do spawn, never sell, trade or give away the offspring, and never release them into the wild. Plan for what you will do with them before eggs appear, since a spawn can produce dozens of fish.

How Do GloFish Danios Breed?

Danios spawn in groups, and the same steps work for most egg-scattering fish. This is how they behave when kept well:

1. Set up a separate tank. Adults eat their own eggs, so use a breeding tank or a spawning box.
2. Condition the fish. Feed live or frozen foods such as mosquito larvae and bloodworms, along with good flakes, for a week or two.
3. Protect the eggs. Cover the bottom with marbles or glass beads, or use a dense mat of Java moss, so the eggs fall out of reach.
4. Use shallow, slightly cooler water. A cooler water change is a common spawning trigger. Spawning usually happens the next morning.
5. Remove the adults. Take them out as soon as you see eggs, and the females have slimmed down.

If the fish don’t spawn, they are usually not conditioned well enough, or the group has no clear male-to-female balance. Males are slimmer, and females are rounder and thicker in the belly. Feed more for another week and try again.

The GloFish species do not interbreed with each other, since they are different species.

Read Related Article: Axolotl Breeding

Tank Requirements For Breeding GloFish

Zebra danios do best in water from the mid-60s to the high 70s °F with a pH of 6.0 to 8.0 and moderate hardness. A group of danios needs a tank of about 20 gallons or more, and a breeding tank needs only to be big enough to hold the adults briefly.

A marble substrate is simply a layer of marbles that the eggs fall between, so the adults can’t reach them. Use a water conditioner on tap water, and run a gentle filter. A sponge filter is safest around eggs and fry.

Feed fry infusoria or powdered fry food at first, and move on to baby brine shrimp and microworms as they grow.

Read Related Article: Breeding Corydoras

GloFish Fun Facts

1. The first GloFish were made in Singapore around 1999. They went on sale in the US in 2003.
2. The fish glow because they carry fluorescent protein genes. Under blue or UV light, the colors are much brighter.
3. GloFish come in several trademarked colors, such as Electric Green, Starfire Red, Sunburst Orange, Cosmic Blue, Galactic Purple and Moonrise Pink.
4. Apart from the glow, they are ordinary fish. They need the same care as the unmodified versions of their species.
5. Most are omnivores and take flakes, small pellets and live or frozen foods.