Panther knifefish

Where they come from

Panther knifefish (Rhamphichthys pantherinus) are South American river fish. Think big, warm, tannin-stained systems with lots of structure, soft bottoms, and a steady supply of worms and small prey in the dark. They are one of those fish that makes total sense once you see how they hunt at night.

Setting up their tank

If you try to keep this fish like a hardy community oddball, you will hate the experience. Give it space, calm water quality, and lots of cover, and it becomes a really cool, confident night hunter.

• Tank size: I would not bother under 125 gallons, and bigger is better if you want tankmates. They are long fish and they like room to turn without scraping.
• Footprint over height: a long 6-foot tank is way nicer than a tall one.
• Substrate: fine sand or very smooth small gravel. They cruise low and can beat up their underside on sharp stuff.
• Hides: big PVC tubes, driftwood tangles, and shaded caves. Give at least 2-3 legit hide options so it can pick a favorite.
• Lighting: dim, with floating plants or tinted water (botanicals) if you like that look.
• Flow and filtration: solid filtration, but not a washing machine. They handle current, but they relax more in moderate flow with calm zones.

Water-wise, aim for warm and on the softer side. Stability matters more than chasing a specific pH number. If your tap is hard, you can still keep them, but you will have an easier time if you can dilute with RO or rainwater and keep nitrates low.

• Temperature: 77-82F (25-28C)
• pH: roughly 6.0-7.2 is a comfortable zone
• Hardness: soft to medium
• Ammonia/nitrite: always 0
• Nitrate: keep it low with water changes and not overfeeding

What to feed them

These are hunters. A lot of them arrive skinny because they were offered flakes and hoped for the best. Once they lock onto real food, they put on size fast.

• Best starters: live blackworms, chopped earthworms, live or frozen bloodworms, and frozen mysis
• Staples once settled: earthworms (my top pick), shrimp pieces, strips of fish fillet, quality carnivore pellets if you can convert them
• Occasional treats: live ghost shrimp or small feeder shrimp (not goldfish or rosy reds)

Feed after lights out or at least at dusk. Mine learned the routine and would come out when the room lights dimmed. If you have bold tankmates, use tongs or a feeding tube so the knifefish actually gets its share.

How they behave and who they get along with

Panther knifefish are mostly peaceful in the sense that they do not go looking for fights. The problem is their mouth and their job description. If it fits, it is food. If it looks like a rival knifefish, it may get bullied.

• Good tankmates: larger calm characins, bigger peaceful cichlids, medium-large catfish that will not harass it, large peaceful plecos
• Risky: fin-nippers (they will stress it nonstop), super aggressive cichlids, anything small enough to be eaten
• Not recommended: other knifefish unless you have a very large tank and a plan for separation

They are more confident if they have a dedicated hide and the tank is not a bright, bare glass box. Expect them to be a lot more active at night. You will see that slow, backward-and-forward hovering and sudden strikes once they settle in.

Breeding tips

Breeding Panther knifefish in home aquariums is rare. Sexing is not straightforward, and getting a compatible pair plus the right seasonal cues is a whole project. Most hobbyists keep them as showy predators rather than breeding goals.

If you want to try anyway, the best path is starting with a group of juveniles in a very large tank, feeding heavy on worms, and mimicking seasonal changes: warmer temps, lots of clean water, and then a big series of water changes that simulate a rainy season. Plenty of cover and a soft sandy bottom help.

Common problems to watch for

Most issues with this fish trace back to two things: dirty water and starvation (usually because it never switched to prepared foods or got outcompeted at feeding time).

• Refusing food after purchase: very common. Start with live blackworms or chopped earthworms in the evening and keep the tank calm.
• Skin damage and infections: they can scrape themselves on rough decor or get burned by ammonia/nitrite. Smooth decor and clean water fix most of it.
• Ich and other parasites: they can show it after shipping stress. Raise temp carefully and treat with meds that are safe for scaleless fish, and watch oxygen levels.
• Jumping: almost always a lid issue or a nighttime scare (sudden light, banging, aggressive tankmate).
• Slow wasting: often internal parasites or chronic underfeeding. A deworming plan plus higher-calorie foods can turn this around if caught early.

My biggest practical advice: watch the belly line. A healthy Panther knife has some thickness behind the head and along the body. If it starts looking like a ribbon, do not wait. Fix feeding, reduce competition, and check for parasites before it gets too weak.