glass knifefish

Where they come from

Glass knifefish in the genus Distocyclus are from the Guiana Shield region of northern South America. Think shaded forest creeks and slow side channels in French Guiana and nearby drainages. The water there is usually soft and slightly acidic, with a tea tint from leaf litter. They cruise through tangles of roots using a weak electric field to feel their way around.

Setting up their tank

Plan for an adult around the foot-long mark. A single fish does well in a 55-75 gallon tank with a tight lid. Keep light low, use fine sand, and give them a maze of hides. Mine spent daylight hours parked in a tube and came out like a ghost at dusk.

• Tank size - 55-75 gallons for one, larger if you try more than one
• Substrate - fine sand or very smooth gravel to protect the skin
• Hides - multiple PVC tubes (1.5-2 inch), driftwood tunnels, stacked slate, leaf litter
• Plants - hardy low-light plants (Anubias, Java fern) and floaters to dim things
• Filtration - strong biofiltration with gentle return; cover intakes with foam
• Flow - gentle to moderate; they do not like to be blasted
• Lid - tight-fitting with blocked gaps; they can and will jump
• Lighting - dim; use floaters or shaded areas
• Temperature - 76-82 F (24-28 C)
• pH and hardness - roughly 5.5-7.0 pH, soft to moderately soft

Keep water steady. These fish sulk and stop eating if ammonia or nitrite creep up or if you swing pH. I had best results with weekly 25-40% water changes, matched for temp and TDS.

What to feed them

They are micropredators. New arrivals usually ignore flakes and hard pellets. Start with soft, meaty foods and switch them over slowly. Feed at dusk or use a dim room and they will come out more confidently.

• Live or frozen blackworms (their favorite starter)
• Frozen bloodworms and mosquito larvae
• Mysis and enriched brine shrimp
• Chopped earthworms or small prawn pieces
• Soft sinking carnivore pellets or gel foods once they are eating well

How they behave and who they get along with

Shy at first, mostly active at night, but they settle into a routine and will start cruising for food at lights-out. They are not aggressive in the usual sense, but they do spar electrically and with short chases around favorite hides.

• Good tankmates: peaceful, medium-sized characins that are too big to swallow, hatchetfish, calm cichlids like keyholes, geophagine types that are not pushy, larger Corydoras.
• Borderline: dwarf cichlids during breeding, boisterous barbs.
• Avoid: fin-nippers, large aggressive cichlids, predatory catfish, shrimp, tiny tetras that can be eaten.

Expect them to claim specific tubes. Give each potential claimant two or three options and you get fewer squabbles.

Breeding tips

This species has not been reliably bred in home aquaria as far as I know. Gymnotiforms often use seasonal cues to spawn. If you want to experiment, you would need a very large, quiet setup, a group raised together, very soft acidic water, tons of cover, and heavy feeding with live foods while simulating a rainy season pulse.

• Condition a group separately on rich live and frozen foods.
• Lower TDS and pH gradually with clean RO-mixed water, then do a series of slightly cooler water changes to mimic rains.
• Provide long tubes and vertical surfaces; some related fish attach eggs in shelters.
• Keep lighting very dim and avoid disturbances.

Common problems to watch for

• Not eating: new fish often refuse food under bright lights or in bare tanks. Dim things and offer live blackworms to get them started.
• Skin scrapes and fungus: they abrade easily on rough decor and filter intakes. Use smooth wood/rocks and foam over intakes.
• Ich and meds: sensitive to harsh treatments. Use gentle heat and carefully chosen meds at reduced dose, lots of aeration.
• Bloat from rich foods: vary the diet and keep portions small but frequent.
• Jumping: they launch during spooks or at lights-on. Secure the lid and avoid sudden light changes.
• Water quality swings: they react badly to ammonia, nitrite, and big TDS jumps. Test regularly.
• Electrical stress: mixing with other electric species or housing multiple knives in tight quarters can lead to constant EOD jamming and hiding.