Fish That Start With O
Browse all aquarium fish species with common names beginning with "O". Each profile includes care requirements, water parameters, tank size recommendations, and compatibility information for freshwater, marine, and brackish species.
The letter 'O' in our species index features a diverse range of aquarium fish, including popular community members like the Golden otocinclus (Otocinclus affinis) and the Common otocinclus (Otocinclus vittatus), as well as the intriguing Harlequin filefish (Oxymonacanthus longirostris). Whether you're looking for efficient algae eaters or unique tank additions, this section offers valuable insights into these captivating species.
Oblique-swimming triplefin
Forsterygion maryannae
A small New Zealand triplefin found over rocky reefs (reported ~1–50 m). It is unusual among triplefins for schooling in the water column above reefs and feeding on planktonic crustaceans (e.g., copepods/euphausiids), often holding an oblique body posture.
Ocean surgeonfish
Acanthurus tractus
Acanthurus tractus is a Western Atlantic tang that cruises reefs in little groups, spending most of the day mowing down benthic algae. It is got that classic surgeonfish attitude (and the tail scalpel to match), so it likes real swimming room and steady, clean reef conditions.
Ocellaris clownfish
Amphiprion ocellaris
Ocellaris clowns are that classic orange clownfish look-three white bars, a little black edging, and a ton of attitude packed into a small fish. They'll "pick a spot" in the tank (often a corner or a coral) and do that cute hover-wiggle thing, and a bonded pair will usually settle in fast and act like they own the place.
Ocular coralblenny
Ecsenius oculatus
Ecsenius oculatus is a tiny little reef-percher from the Christmas Island/Western Australia area that spends its day scooting between holes and ledges and watching you like it owns the place. It is an algae-and-film grazer by nature, so in a mature reef it will constantly pick at rocks and glass and do that classic blenny hover-and-hop routine.
Oluolus hatchetfish
Polyipnus oluolus
Polyipnus oluolus is a tiny deepwater marine hatchetfish from the Marshall Islands that lives out in the open ocean and uses little light organs (photophores) on its body for camouflage and signaling in the dim water. It is super cool from a biology standpoint, but its pelagic deep-sea lifestyle makes it basically a non-aquarium species for normal hobby setups.
Oman snake eel
Yirrkala omanensis
This is a little snake eel from the Gulf of Oman that lives down on the bottom and does the classic eel thing - hiding/burrowing and staying out of sight most of the time. Its not really an aquarium-trade species, and most of what we know is from scientific collection records rather than hobby care notes. If you ever did try one, think secure lid and a sand-friendly setup, because ophichthids are basically born escape artists and diggers.
Onesided livebearer
Jenynsia lineata
A quirky South American livebearer that hugs the mid-to-top of the tank and shows off broken stripe patterns. Females get much bigger than the little males, and their odd one-sided mating gear is a fun bit of fish trivia to share. Give them cooler, clean water and a small group and they settle in nicely.
One-spot Jupiaba tetra
Jupiaba mucronata
Jupiaba mucronata is a little Guyana characin that tops out around 4 cm SL, and it fits that classic "busy, always on the move" tetra vibe. Its claim to fame in a lot of checklists is the single prominent shoulder (humeral) spot, and it does best when you lean into a roomy, current-friendly setup and keep it in a proper group.
Onestriped livebearer
Jenynsia unitaenia
Jenynsia unitaenia is a small freshwater onesided livebearer from coastal rivers/streams in southern Brazil (Santa Catarina and northeastern Rio Grande do Sul). It is identified by a single midlateral stripe.
Orange bellowfish
Notopogon fernandezianus
Notopogon fernandezianus is the orange bellowfish, a weird little deepwater "trumpet fish" with a long snout and a tall, humped body. It lives way down on the continental shelf and slope (roughly 150-580 m), so its natural world is cold, dark, and high-pressure - basically the opposite of a home aquarium. Super cool to look at, but not a realistic species to keep alive long-term in normal hobby setups.
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Orange chromide
Pseudetroplus maculatus (syn. Etroplus maculatus)
This is that cute little Indian/Sri Lankan cichlid with the big black "shoulder" spot and a warm gold/orange glow when it's happy. It'll do the classic cichlid thing where it gets a bit pushy when breeding, but most of the time it's pretty chill-especially if you keep a small group. Super cool bonus: the parents actively tend the eggs and fry, and the babies even graze on the parents' skin mucus for a bit.
Orange-spotted toadfish
Torquigener hypselogeneion
This is a small Indo-west Pacific puffer that hangs around sandy flats and estuaries, and it will literally bury itself in the sand to nap with just the eyes sticking out. Its cheek bars and orange-yellow spotting make it look like a little camo tank. Cool fish to read about, but its pufferfish teeth and potential toxicity mean its not a typical community-aquarium pet.
Oriental bluespotted maskray
Neotrygon orientalis
Neotrygon orientalis is a smallish bluespotted maskray from the Indo-Malay/Philippines region - a bottom-hugging marine ray that cruises sandy areas and reefy flats. Its disk is sprinkled with blue spots and it has that classic "mask" marking around the eyes, but its real "wow" factor is how much space and clean sand it needs to live well. This is one of those animals that gets mislabeled as "a stingray for big home tanks" when it really belongs in public-aquarium-level setups.
Ornate fin nipper
Ichthyborus ornatus
This is one of those Congo oddball characins that looks like it should be a fin-nipper, but it is actually built to hunt and swallow smaller fish whole. The bold tail pattern is super distinctive, and it is a really cool pick if you like predator tanks and want something different than the usual cichlids.
Osteochilus kerinciensis
Osteochilus kerinciensis
This is a mid-sized Southeast Asian cyprinid from Sumatra, and its whole world basically revolves around the Batang Hari drainage (including highland lakes like Lake Kerinci). Its wild range is pretty tight, and there is basically no solid aquarium-specific info out there for it, so if you ever see one in the trade its best to treat it like a riverine labeonin barb: clean water, lots of oxygen, and a calmer community setup with room to cruise.
Ouachita shiner
Lythrurus snelsoni
This is a little Ouachita Mountains native shiner that stays genuinely small (around 2 inches max), so its all about a tight school and lots of open swimming room. Like other Lythrurus, it can really color up when its happy and settled, especially if you keep it cool, clean, and in a group. Its not a hard fish once established, but its way less forgiving of warm, low-oxygen, dirty conditions than most beginner tropicals.