Blackfin pupfish
Cyprinodon beltrani
Also known as: Cachorrito lodero
Cyprinodon beltrani is a tiny Mexican pupfish from Lake Chichancanab, and the males get those dark fin accents that make them look way tougher than their size. They are busy little substrate-pickers (detritus and other tiny bits), and they can be surprisingly feisty with each other, especially male-to-male.
The Blackfin pupfish exhibits a metallic blue hue with a distinctive black dorsal fin and elongated body, measuring up to 5 cm in length.
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Quick Facts
Size
5 cm
Temperament
Semi-aggressive
Difficulty
Advanced
Min Tank Size
20 gallons
Lifespan
3-5 years
Origin
Central America (Mexico - Yucatan Peninsula)
Diet
Omnivore/detritivore - quality small pellets/flakes, algae-based foods, frozen/live foods (brine shrimp, daphnia), lots of grazing
Water Parameters
22-28°C
7-8.5
10-25 dGH
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Calculate heater sizeCare Notes
- Give them a long tank with lots of open swimming room and a few rock piles or dense plants to break up sight lines - males will claim little territories and bully hard in a bare box.
- They do best in hard, alkaline freshwater (think pH 7.5-8.5, GH 10+), and they crash fast if ammonia or nitrite shows up, so skip the brand-new tank and run a seasoned filter.
- Keep it warm but not tropical: 72-78F is a comfy range, and steady matters more than chasing a number - big daily swings make them sulky and prone to disease.
- Feed small amounts 2-3 times a day; I rotate quality flakes/pellets with frozen foods like brine shrimp, daphnia, and mosquito larvae to keep color and breeding behavior strong.
- Avoid slow or long-finned tankmates (they nip) and avoid anything bigger and pushy (they get shredded); best is a species tank or tough, fast fish that like hard water, like livebearers.
- If you want breeding, set up a trio (1 male, 2+ females) with fine-leaf plants or a spawning mop - they scatter eggs and the adults will snack on them if you leave them together.
- Watch for constant chasing, clamped fins, and torn tails - if one male is terrorizing the tank, add more cover, remove the bully, or split them because they do not "work it out".
Compatibility
Good Tankmates
- Fast, tough livebearers like mollies and platies - they can handle the pupfish attitude, stay in the mid-upper water, and dont get too stressed by chasing
- Smaller rainbowfish (Melanotaenia praecox and other similarly sized rainbows) - active swimmers that dont take it personal if a male pupfish gets pushy
- Hardy barbs that are not super aggressive (checkered barbs or cherry barbs in a decent group) - they move quick and usually dont get pinned into a corner
- Bottom crews that keep to themselves like Corydoras or small/medium plecos - pupfish mostly squabble in the open water, so bottom fish are usually ignored
- Loaches that like to scoot around (kuhli loaches) - they are out of the pupfish line of fire most of the day, and they are quick when they do come out
- Other Blackfin pupfish - best in a bigger group with lots of sight breaks (plants, rocks) and a skewed ratio like 1 male to 2-3 females so one fish doesnt get constantly targeted
Avoid
- Slow fish with fancy fins like bettas, guppies with long tails, and fancy goldfish - the pupfish will often test-bite and chase, and the slow ones cant get away
- Very peaceful, small community fish like neon tetras, ember tetras, and celestial pearl danios - they tend to get stressed and can get their fins picked at
- Other semi-aggressive brawlers like convict cichlids, jewel cichlids, and bigger Central American cichlids - it turns into nonstop sparring or somebody gets wrecked
Where they come from
Blackfin pupfish (Cyprinodon beltrani) are one of those desert fish that make you do a double take. They come from northern Mexico, living in spring-fed waters where conditions can swing hard - heat, minerals, seasonal changes, you name it. That background is why they are tough in some ways, but also why they punish sloppy setups.
If you have only kept community fish that like stable, soft water, this is a different game. Think hardy fish with an attitude, from hard-water habitats.
Setting up their tank
Give them floor space more than height. They are active and territorial, and you will see way better behavior in a longer tank. I would not do these in a tiny desktop setup unless its a species-only project and you are really on top of maintenance.
- Tank size: 20 long as a starting point for a small group, bigger is easier
- Filtration: sponge filter or a gentle HOB with a prefilter sponge (they can be scrappy but they do not need a river)
- Substrate: sand or fine gravel so you can keep it clean and not trap gunk
- Hardscape: lots of line-of-sight breaks (rocks, wood, dense plant clumps) so the weaker fish can get out of trouble
- Plants: tough stuff like Java fern, Anubias, or floaters - they can handle hard water and bright light
I keep them in hard, alkaline freshwater. They handle warm water fine, but I have the best luck keeping them in the low-to-mid 70s F so they are active without constantly acting like they are in breeding mode. Stability matters more than chasing an exact number.
Build the tank around territories. If you only add one rock and one plant, the dominant male will claim the entire aquarium. Give them multiple "corners" and visual barriers so the heat gets spread out.
They are jumpy, especially during chasing. Use a lid. Tiny gaps around hoses and cords are exactly where a pupfish will find a way out.
What to feed them
These guys eat like little pigs. In nature they pick at small inverts, algae, and whatever drifts by. In a tank, they do best with variety, and they color up better when you do not run them on one food.
- Staple: a good quality small pellet or flake (I like something with a bit of plant content, not just pure protein)
- Frozen: brine shrimp, daphnia, cyclops, chopped bloodworms (bloodworms are a treat, not the whole diet)
- Live: baby brine shrimp, grindal worms, moina/daphnia if you culture them
- Grazing: they will pick at algae and biofilm, so do not keep the tank "sterile"
Feed small amounts 1-2 times a day. If you dump in a big meal, the dominant fish stuffs itself and the shy ones get ragged from chasing.
How they behave and who they get along with
Pupfish are not peaceful community fish, even though they are small. The males posture, chase, and defend little patches like they are cichlids. You will see lip-locking, fin-nipping, and nonstop sparring if the setup is too open or the group is too small.
I have the best results keeping them as a species group. If you really want tankmates, think fast, tough, and not flashy-finned. But honestly, most "good" community fish either get harassed or outcompeted at feeding time.
- Best option: species-only group
- Group structure: one male with several females can be calmer than multiple males in a small tank
- If keeping multiple males: add more space and more visual barriers, and expect some fin wear
- Avoid: slow fish, long fins, shy fish, and anything you would be upset to see nipped
Watch for one fish getting pinned in a corner or hiding all day. With pupfish, "a little chasing" can turn into a death sentence if the tank is too bare.
Breeding tips
If you keep them happy and fed, they will usually breed without you doing anything fancy. The tricky part is keeping fry alive, because adults will snack on eggs and tiny babies whenever they find them.
- Spawning setup: a mop (yarn mop) or dense fine plants where eggs can fall out of reach
- Conditioning: more live/frozen foods for a couple weeks, plus frequent water changes
- Egg handling: pull the mop every day or two and hatch eggs in a small container with gentle aeration
- Fry food: baby brine shrimp is the easy button; microworms can help early on
- Grow-out: separate by size if you have bullies or fast growers
If you want to raise numbers, do not rely on "natural" survival in the main tank. A simple mop rotation (one in the tank, one being checked) makes a huge difference.
Common problems to watch for
Most issues I have seen with pupfish come from two things: aggression stress and dirty water from heavy feeding. They are eager eaters, so it is easy to overdo it and end up with rising nitrates and funky bottom debris.
- Fin damage from chasing: add cover, reduce male-to-male conflict, and keep water clean so tears do not turn into infections
- Sudden deaths after a big change: avoid big parameter swings, especially if your tap water is very different from the tank
- Bloat/constipation: back off rich foods and add daphnia or a more plant-leaning staple
- Jumping: lid, lid, lid
- Ich and other parasites: stress from bullying makes them more likely to break with spots
Do not ignore bullying because "they are just sparring." If one fish is clamped up, hiding, or getting shredded, pull the aggressor or rearrange the tank the same day.
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