Top 5 Oddball Fish for A 20-Gallon Aquarium

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 hatchetfish is a tiny species measuring 1.25 inches (3 cm). It has a prominent chest, resembling a hatchet knife. The pearly, light-colored body has beautiful, dark marbling. Its pectoral fins are shaped like 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. Small mouths make it difficult to feed these fish small floating foods like Easy Fry and Small Fish Feed, crushed flakes and baby brine shrimps 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. Similar to otocinclus catfish they are aufwuchs grazers. They eat any algae, zooplankton or biofilm that grows on surfaces. Their favorite foods include Repashy Soilent Green, frozen daphnia, baby brine shrimp, algae wafers, and canned green beans. Stiphodon gobies can be great community fish. The males can sometimes be a bit aggressive towards each other. Provide plenty of hiding spots for them and consider getting more females than boys.

3. Peacock Gudgeon

Tateurndina ocellicauda

Because of its incredible rainbow colors, the peacock gudgeon is commonly known as the “peacock gudgeon”. 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 an obvious nuchal hump on the forehands, as seen in the photo above. While the females have more of a streamlined forehead, much like a typical tetra and danio, They are native to Papua New Guinea, and they prefer to live in the lower part of the aquarium. However, they don’t have a preference for particular foods and will happily eat all floating and sinking community food items you provide. Although they are generally calm-mannered, males can become territorial in 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

This species can be found in two types in nature. One is a regular version that lives in rivers and lakes and looks similar to a silvery tetra, and the other is a blind cave version that lives underground in Mexico’s caverns and waterways. This latter variety is more common in aquarium hobby due to its shiny, pinkish body with undeveloped eyes that are covered by skin. They can still find food due to their improved senses of taste and smell. They also have the ability to navigate using their lateral lines, which 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. Although they are considered a good community fish, they can be aggressive and will nip at anything to explore their surroundings. Keep them away from slower-moving fish or those with long fins. Keep them happy with a variety community food like pellets, gel foods, and flakes.

5. Top Hat Blenny

Omobranchus fasciolatoceps

Blennies are most commonly found in saltwater habitats. This is unfortunate for freshwater hobbyists since they have such interesting personalities. There are some species that can live in brackish waters, such as the top-hat blenny from China and southern Japan. While they are often called a freshwater blenny by some, their preference is for brackish water that has a higher pH and GH. Their common name refers to a male with a round crest on their head. The entire head and face of the males are covered in a striking pattern vertically striped yellow. The rest of their 2.5 to 3 inch (6-8 cm) long, elongated bodies is brown or grayish in color and ends with a yellow tail. They are usually peaceful fish. But males may fight over territory so they need rockwork and caves to shelter them. You can find them eating brine shrimp, baby salt shrimp, dried seaweed and spirulina flakes.

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.