X-ray tetra

Where they come from

X-ray tetras (Pristella maxillaris) come from northern South America—think the Amazon/Orinoco region and nearby coastal rivers. A lot of what we buy are tank-bred these days, which is great because they’re usually pretty adaptable and forgiving in typical community-tank water.

Setting up their tank

These are easygoing schooling fish, but they look and act way better when you give them some room and some cover. I like them in a 20 gallon long or bigger, especially if you want a proper group instead of a sad little trio.

• Group size: 8–12 is the sweet spot. They calm down, color up, and spread any bossiness around.
• Tank size: 15–20 gallons minimum for a decent school; bigger if you’re building a full community.
• Filtration: gentle to moderate flow. A sponge filter works great, or a HOB with a prefilter sponge on the intake.
• Decor: plants (real or fake), driftwood, and some open swimming lane through the middle.
• Lighting: medium is fine. With plants and a darker substrate, they really pop.

Water-wise, they’re not fussy. If your tank is stable, they’ll roll with a pretty wide range. I’ve kept them in slightly soft to moderately hard water without drama. Temperature in the mid-70s°F (around 24–26°C) keeps them active and hungry.

What to feed them

X-ray tetras aren’t picky eaters. They’re classic “tiny vacuum cleaners,” and they’ll take most community foods right away. The only trick is making sure everyone gets a bite, because they can be sneaky fast at mealtime.

• Daily staple: a good quality micro pellet or small flake
• Boosters: frozen brine shrimp, daphnia, cyclops, or bloodworms (small portions)
• Live foods (if you do them): baby brine shrimp or daphnia are like candy
• For color and condition: rotate foods instead of feeding the same thing every day

Behavior and tankmates

They’re peaceful, busy little swimmers. In a good-sized group they’ll cruise the mid-water, spar a bit (nothing serious), and show off those yellow/black fin markings. If you keep too few, they can get nippy out of boredom or stress—usually at each other, sometimes at slow-finned neighbors.

• Great tankmates: corydoras, small rasboras, other peaceful tetras, otocinclus, snails, shrimp (adult shrimp usually fine)
• Also works: dwarf gourami/honey gourami (watch for individual temperament), small peaceful plecos
• I’d avoid: long-finned show fish (fancy guppies, bettas) if you notice fin curiosity, and anything big enough to see them as snacks

Breeding tips (if you want to try it)

They’ll breed in home tanks, but you usually won’t get many surviving fry in a community because the adults will snack on eggs and babies. If you actually want to raise them, a simple little breeding setup makes it way easier.

• Use a small separate tank (5–10 gallons) with a sponge filter
• Add a spawning mop or dense fine plants (java moss works)
• Dim the light and keep the bottom bare so you can clean easily
• Condition the adults for a week with frozen/live foods
• Move a pair or small group in the evening; spawning often happens early next day
• Pull the adults after you see eggs, or use a mesh/marbles so eggs drop out of reach
• First foods for fry: infusoria/liquid fry food, then baby brine shrimp as they grow

Common problems to watch for

Most issues I’ve seen with X-ray tetras come down to stress from a small group, a bare tank, or a new setup that’s still swinging around. They’re hardy, but they’re still tetras—dirty water and sudden changes catch up with them.

• Skittish hiding and washed-out color: usually too few in the school, bright/bare tank, or lots of booming footsteps/kids/pets right in front of the glass
• Fin nips or ragged fins: small group, crowding, or pairing them with slow long-finned fish
• Ich after adding new fish: common tetra problem—quarantine new arrivals if you can
• Mouth fungus/columnaris-looking fuzz: often tied to stress + warm water + poor water quality; treat quickly and improve water conditions
• Sudden losses in a new tank: check for ammonia/nitrite spikes and slow down feeding

If you give them a stable, cycled tank and a real school, X-ray tetras are one of those fish that just quietly succeed. They’re cheap, tough, and they add a ton of movement without bullying the rest of your community.