Top 5 Oddball Fish for a 20-Gallon Aquarium

Looking for a fish that is bizarre in appearance, has unique behaviors, or is rarer in availability? In the aquarium hobby, we have a whole category dedicated to these “oddball fish.” Some species are quite hardy and easy to keep, while others have specialized care requirements to accommodate their unusual physiology. Learn about 5 of the coolest oddballs that you can keep in a 20-gallon aquarium.

1. Marbled Hachetfish

Carnegiella strigata

This 1.25-inch (3 cm) species is one of the smallest hatchetfish and has a prominent chest that looks like a hatchet blade. Its pearly, light-colored body is covered in beautiful, dark marbling, and the pectoral fins stretch out like little wings. They come from tannin-filled black waters in the Amazon basin of South America, which tends to have acidic pH and tropical temperatures, but they are accustomed to handling a wide range of water parameters because the area is subject to annual flooding.

Because hatchetfish are top-dwelling fish that are capable of jumping out of the water to escape predators, make sure to have a tight-fitting lid on the aquarium and cover any little openings with craft mesh or other materials. To help them feel more comfortable, add floating plants like water sprite as shelter and get a school of at least six marbled hatchetfish because the more the merrier. This species pairs well with other peaceful community fish that swim in the middle and bottom layers of the aquarium. Because of their small mouths, feed tiny floating foods like crushed flakes, Easy Fry and Small Fish Food, baby brine shrimp, and daphnia. For more details, see our hatchetfish care guide.

2. Stiphodon Goby

Stiphodon ornatus

The Stiphodon genus consists of freshwater gobies from Asia and Oceania that have a slender, eel-like body similar to kuhli loaches but with half the length at around 2 inches (5 cm) long. Like otocinclus catfish, they are aufwuchs grazers, meaning they like to constantly forage on algae, zooplankton, and biofilm found growing on surfaces. Their favorite foods include Repashy Soilent Green, frozen daphnia, baby brine shrimp, algae wafers, and canned green beans. While Stiphodon gobies are excellent community fish, the males can sometimes be a bit feisty towards one other, so provide plenty of hiding spots and consider getting more girls than boys, even though the females are less colorful.

3. Peacock Gudgeon

Tateurndina ocellicauda

The peacock gudgeon gets its common name from its amazing array of rainbow colors. Imagine a 2.5-inch (6 cm), pink body with red vertical stripes, blue speckling, yellow-rimmed fins, and a black spot at the base of the tail. The males have a distinct nuchal hump on their forehands (as seen in the picture above), whereas females have a more streamlined forehead much like your typical tetra or danio. They come from Papua New Guinea and tend to dwell in the lower half of the aquarium. That being said, they are not picky eaters and are happy to consume any floating and sinking community fish foods you feed them. While they are normally mild-mannered fish, males can become territorial during breeding seasons. They are quite easy to spawn if you have both sexes and provide 1-inch (2.5 cm) PVC pipes for them to lay their eggs.

4. Blind Cave Tetra

Astyanax mexicanus

In nature, this species actually comes in two forms — (1) a normal version found in rivers and lakes that looks like an ordinary, silvery tetra and (2) a blind cave version found in underground waterways and caverns in Mexico. The latter type is more popular in the aquarium hobby because of its shiny, pinkish body and undeveloped eyes covered by skin. Despite the blindness, they can easily find food with their enhanced senses of smell and taste and navigate by using their lateral lines to detect changes in water pressure. This resilient schooling fish grows to around 3–3.5 inches (8–9 cm) and can live in a cool water aquarium with no heater. While they are rated as a community fish, be aware that they like to nip at things to investigate their surroundings, so keep them away from slow-moving or long-finned fish. Offer a variety of community foods like flakes, pellets, gel food, and frozen foods to keep them happy and well-fed.

5. Top Hat Blenny

Omobranchus fasciolatoceps

Blennies predominently come from saltwater habitats, which is a shame for freshwater hobbyists because they have such fun personalities and interesting behaviors. Luckily, there are some species that live in brackish water, like the top hat blenny that comes from southern Japan and China. They are often marketed as a freshwater blenny, but in our experience, they do best in brackish water with alkaline pH, higher GH, and tropical temperatures. As per their common name, males have a rounded crest atop their head, and their entire head and face has a striking pattern of vertical, yellow striping. The rest of their 2.5- to 3-inch (6–8 cm) elongated body is a brown or grayish color that ends in a yellow tail. Generally speaking, they are peaceful fish, but males will spar with their own kind over territory so provide rockwork and caves for them to seek shelter. Their favorite foods include brine shrimp, baby brine shrimp, dried seaweed, spirulina flakes, and even algae you scrape off the tank sides.

If you love the idea of oddball fish but only have the room to set up a 10-gallon aquarium, then don’t forget to check out our previous article that covers some of our favorite nano-sized picks.