Parasitic catfish

Where they come from

Ochmacanthus alternus is one of the candiru-type parasitic catfish from South America. Youre looking at a fish that naturally hangs around big river systems (think Amazon and its tributaries), hiding in structure and keying in on other fish by smell and movement. In the wild theyre built for quick hit-and-run feeding, not cruising around looking for flakes.

Setting up their tank

If you try to display this fish like a community oddball, youll probably just end up with a stressed, skinny candiru and a bunch of battered tankmates. I had the best results treating the tank like a quiet holding system: stable water, lots of cover, low drama.

• Tank size: bigger is easier, but not for swimming room - for dilution and stability. Id call 30-40 gallons a bare minimum for a single specimen, more if youre also keeping host fish.
• Filtration: strong and reliable, with gentle flow areas. They dont need a river blast, but they do need clean water because youre usually feeding heavy/protein-rich foods.
• Layout: piles of rounded wood, leaf litter, caves, and shaded spots. Give them places to wedge into and disappear.
• Substrate: sand or fine gravel. Theyre not big diggers, but sand is easier on them when they dart and scrape around.
• Lighting: dim. Bright tanks keep them edgy and hiding nonstop.
• Cover: tight lid. They can bolt when spooked, especially during netting and water changes.

Water-wise, youre aiming for warm, soft-ish, and clean. I kept them in the mid-70s to low-80s F and they handled it well. The exact pH number matters less than keeping it steady and not letting waste build up. Frequent partial water changes beat big, infrequent ones with this fish.

What to feed them

This is the part that makes them expert-level. In nature, Ochmacanthus alternus feeds by attaching to other fish and taking mucus/tissue/blood. In an aquarium, getting them to eat non-living foods can be hit or miss, and a lot of individuals just dont make the switch.

If you do want to try, start with the most tempting, meaty options and feed at lights-out. I had the best response with fresh or thawed items that have a strong scent. Some individuals will learn to take food from tongs, but plenty never do.

• Try first: live blackworms, chopped earthworms, live shrimp, or very fresh/thawed bloodworms (not the dusty old cube).
• Also try: strips of fish flesh (tilapia-type), raw shrimp bits, and other high-protein marine/fresh seafood pieces.
• How to offer: long tweezers near their hiding spot after dark, or place food in a shallow dish so it doesnt vanish into the substrate.
• Frequency: small offerings more often. Big dumps just foul the water and stress the tank.
• Watch the belly: a constantly pinched-in abdomen is your early warning sign.

How they behave and who they get along with

Theyre sneaky, mostly nocturnal, and they dont act like a typical catfish. Daytime youll barely see them. At night they get bold and start cruising low and fast, testing anything that looks like a meal.

Tankmates are tricky because their whole strategy is attaching to other fish. Even if theyre ignoring everyone today, that can change the moment theyre hungry or stressed.

• Best option: species-only or a dedicated setup built around their needs.
• If you keep other fish: expect risk. Larger, tough-bodied fish are less likely to be killed quickly, but they can still be wounded and infected.
• Avoid: slow fish, scaleless fish (loaches, many catfish), fancy fins, and anything you cant stand to lose.
• Do not mix with: small fish. Theyre not safe and may end up as targets or stressed into disease.

Breeding tips

Real talk: breeding Ochmacanthus alternus in home aquariums is basically undocumented and not something most hobbyists pull off. Theyre not like Corydoras where you can condition them and trigger a spawn with a cool water change.

If you ever wanted to experiment, Id focus on long-term stability, lots of hiding spots, and keeping a small group in a large, mature system. But the bigger barrier is usually keeping them feeding well enough for long enough to even reach a point where breeding is on the table.

Common problems to watch for

Most problems come from two things: starvation and secondary infections (either on the candiru or on the fish it damages). Because theyre small and secretive, people miss the early signs.

• Starvation: pinched belly, lethargy at night, losing that quick darting movement.
• Water quality crashes: cloudy water, sour smell, sudden hiding and rapid breathing. Heavy meaty feeding can overwhelm a tank fast.
• Injuries from netting: they tangle and wedge into mesh. Use a container to move them if you can.
• Parasites/bacteria: wild-caught individuals can arrive with baggage. Quarantine is not optional.
• Host fish infections: red sores, fuzzy patches, or rapid decline after a suspected attachment event.

If you take anything from this: plan the feeding and the ethics first, then build the tank around stability and cover. Theyre fascinating little nightmares, but theyre not forgiving, and theyre not a project you wing as you go.