The females usually have a black stripe near their eye area and light spots can be seen on the caudal and dorsal fins of both sexes.

Pregnant females are also easily recognizable by their gravid spot; a darker area on their bellies where they hold the fry.

[6][7][8] On average, G. holbrooki has one[6][7][8] or two[6] additional dorsal fin rays compared to G. affinis, but this characteristic varies both by sex and by individual in these species.

[7]: Table 1 In eastern Australia, the female and juvenile local Pacific blue-eye (Pseudomugil signifer) are similar in appearance but have a forked tail fin.

[5] The identity of the person honored in the specific name is uncertain but is thought to be the physician and naturalist John Edwards Holbrook (1796–1871).

[2][13][14] Eastern mosquitofish are found in shallow, standing to slow-flowing water, mostly in vegetated ponds, lakes, and sloughs.

[12] The Eastern mosquitofish is easily maintained and has never been considered an endangered or threatened species due to its ability to thrive in its native habitat.

[14] These findings highlight the ecological flexibility of these species, contributing to their extensive success and posing a substantial risk for further range expansion.

[14] Temperature has been shown to change the length of time it takes them to reach reproductive maturity and body size.

Females tend to choose areas of shallow water with dark soil cover for brooding sites, while juveniles prefer more rooted plants in which to hide.

[12] Eastern mosquitofish were introduced into New South Wales in the 1920s to control mosquitoes, but provided no further benefit over small Australian native fish.

Compounding the issue, Eastern mosquitofish have the ability to thrive in many different environmental conditions which are usually lethal to other fish species.

The chemicals used were found to have a strong effect on the G. holbrooki, but they became tolerant to most of them fairly rapidly unless amounts considered unsafe for native species were used.

The main reason it failed was mosquitofish stay in the shallows, which receive the smallest charge from the electrification method used.

In the introduced areas, it has been known to cause top-down trophic effects due to its eating the larvae of some top predators, which include frogs and other fish.