Comet
Calloplesiops altivelis
Also known as: Marine betta, Betta, Merakan
This is the famous "Marine Betta" look-alike: jet-dark with those starry spots, and that wild fake eye near the back that makes predators bite the wrong end. It's a super shy cave-dweller by day and then turns into a sneaky night hunter, cruising out for crustaceans and small fish.
The Comet (Calloplesiops altivelis) features a compressed body with vibrant blue and yellow coloration, highlighted by a distinctive elongated dorsal fin.
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Quick Facts
Size
20 cm (8 inches)
Temperament
Semi-aggressive
Difficulty
Intermediate
Min Tank Size
75 gallons
Lifespan
6-9 years
Origin
Indo-Pacific
Diet
Carnivore/piscivore - meaty frozen foods (mysis, shrimp, chopped seafood), may eat small fish and small ornamental shrimp
Water Parameters
22.2-28°C
8.1-8.4
8-12 dGH
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This species needs 22.2-28°C in a 75 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Give it lots of caves/overhangs and shaded spots-these guys want to lurk, not cruise open water, and they'll stay way calmer when they have a "home base."
- Keep salinity stable (commonly listed around ~1.020-1.025 in care sheets) and maintain high water quality; they may hide/refuse food when stressed by poor or unstable conditions.
- They're basically nocturnal, so feed after lights out or at least at dusk; target-feed with tongs so faster fish don't steal everything.
- Start with meaty foods: mysis, chopped shrimp, squid, and quality frozen blends; if yours is picky, entice with live ghost shrimp for a bit, then switch to frozen on a stick.
- Skip tiny tankmates (small gobies, shrimp, ornamental crabs) unless you're cool with them becoming snacks-if it fits in that mouth, it's on the menu.
- Good neighbors are calm, medium-to-large fish that won't harass it (tangs, dwarf angels, wrasses that aren't bullies); avoid aggressive dottybacks/triggers that will keep it pinned in a corner.
- Watch for "refusing food" after purchase-quarantine if you can and check for flukes/ich; they often look fine but won't eat if something's bugging them.
- Breeding isn't common in home tanks, but pairs have been known to spawn in caves; if you ever see them hanging together in the same den and getting territorial, that's your hint a pair might be forming.
Compatibility
Good Tankmates
- Other mellow, chunky reef fish that mind their own business-think dwarf/medium angels like Coral Beauty or Flame Angel (in a big tank with lots of rock so everyone can claim a spot).
- Rabbitfish (Foxface, One-spot, etc.). They're usually chill, not easily bullied, and they don't act like food the way tiny fish do.
- Tangs in the medium range (Yellow, Kole, Tomini). Active but generally not predatory, and they're fast enough that the comet won't mess with them.
- Medium-to-large, peaceful community fish that won't harass it; avoid very small tankmates that can be eaten and avoid aggressive cave-territorial bullies.
- Bristletooth or other larger blennies (like a Starry Blenny) and bigger gobies that aren't micro-sized. They're tough little characters and usually hold their ground.
- Most non-predatory, medium reef fish with some size to them-chromis/anthias only if they're big enough and you've got lots of hiding spots (comets are sneaky ambush feeders).
Avoid
- Small fish and crustaceans it can fit in its mouth (may be eaten).
- Slow, hover-y fish that sleep in the open-firefish and similar timid, floaty types. Comets love hunting at dusk/night and those guys are easy targets.
- Big bullies and fin-rippers: dottybacks, large damsels, and super-territorial fish that camp the same caves. They'll stress the comet or start constant rockwork wars.
- Other ambush predators in the same niche (lionfish, large hawkfish, big groupers). It turns into a territory/food competition thing, and somebody usually loses.
1) Where they come from
The “Comet” here is Calloplesiops altivelis, better known as the Marine Betta / Comet Grouper. They’re from the Indo-Pacific—reef caves, ledges, and shadowy overhangs. That background explains basically everything about them in captivity: they like dimmer zones, tight hideouts, and they do a lot of their living after lights-out.
2) Setting up their tank
Give this fish a tank that feels like a cave system, not an open swimming pool. Mine spent the first week wedged into one specific rock crack, then slowly started doing confident loops once it decided the place was “safe.”
- Tank size: I’d start at 55g for a single adult, more if you want bulky tankmates.
- Aquascape: lots of rock with real tunnels/overhangs. PVC elbows hidden behind rock work great as “backup caves.”
- Lighting: they don’t need dim lighting, but they appreciate shaded areas. Bright reef lights are fine if you build dark pockets.
- Flow: moderate. They aren’t fans of being blasted in their favorite cave entrance all day.
- Lid: cover the tank. They’re not notorious jumpers like wrasses, but startled fish do dumb things at 2 a.m.
If your Comet is always hiding, don’t panic. Add one more snug hide and reduce foot traffic near the tank for a bit. Once they settle, you’ll see them way more—usually at dusk and feeding time.
They’re surprisingly good at backing into rockwork and getting stuck if gaps are tight. If you build with sharp or pinchy crevices, rearrange it. Smooth “caves” beat jagged cracks.
3) What to feed them
Comets are predators with a big mouth. The main challenge isn’t finding foods they can eat—it’s getting a shy new fish to recognize your food as food. Once they’re converted, they’re solid eaters.
- Great starters: frozen mysis, chopped shrimp, chopped clam, small chunks of squid.
- Once settled: quality frozen blends, larger mysis, and meaty pellets if you can wean them.
- Feeding rhythm: small meals 3–5x/week works well for adults; younger fish do better with smaller portions more often.
- How to deliver: tongs or a feeding stick right near their cave entrance is a cheat code.
Try feeding right as your lights ramp down (or right after they go off). You’ll catch them in “hunt mode,” and they learn faster.
Avoid using live feeder fish. Besides the disease risk, it can lock them into picky habits. You want them on frozen/prepared foods long-term.
4) Behavior and tankmates
They’re calm, a little spooky, and weirdly confident once they own the place. The famous eyespot on the dorsal fin isn’t just decoration—it’s part of their bluff. Mine would “face away” from curious fish and flare that spot like a warning sign.
Tankmate rule is simple: if it fits in their mouth, it’s food. If it bullies them off food, you’ll never see your Comet. Aim for peaceful-to-semi-aggressive fish that won’t harass a cave-dweller.
- Usually fine: tangs, rabbitfish, larger clownfish, peaceful wrasses (not tiny ones), dwarf angels with attitude kept in check.
- Use caution: dottybacks and aggressive damsels (they can terrorize a shy Comet), big triggerfish (may pester), boisterous puffers (fin-nipping).
- Not safe: small shrimp and tiny fish (think small gobies, neon-size stuff), especially at night.
Reef compatibility: they generally ignore corals. The issue is “cleanup crew math”—small ornamental shrimp can disappear, and tiny crabs aren’t guaranteed to last.
5) Breeding tips (realistically)
Breeding Marine Bettas at home is possible, but it’s not a casual weekend project. They’re cave spawners, and pairs can be tricky to form because sexing them is not straightforward and adults can be territorial in tight spaces.
- Best shot: start with two juveniles and let a pair form over time in a larger tank with multiple caves.
- Provide “spawn-friendly” caves: deeper, darker chambers with a single entrance.
- Conditioning: heavier feeding on meaty foods, stable water, and a calm tank tends to get you closer than chasing numbers.
- If you get eggs/larvae: you’re in live-food territory fast (rotifers, copepods, etc.), and it becomes a separate rearing setup situation.
If breeding is your goal, plan the whole system around it from the start. A display tank “maybe they’ll spawn” setup usually ends with… no babies and a fish that just likes its cave.
6) Common problems to watch for
Most Comet issues I’ve seen come down to stress and feeding—either they don’t feel secure enough to eat, or tankmates make feeding chaotic. Once they’re comfortable and eating well, they’re pretty hardy.
- New fish refusing food: usually shyness, bright/open aquascape, or too much competition at feeding time.
- Getting skinny while “eating sometimes”: food is being stolen. Target feed near the cave, or feed after lights-out.
- Crypt/Ich risk: they’re not magic-proof. Quarantine is your friend, especially because treatment in a reef display is a headache.
- Mouth injuries: they can scrape up their face squeezing through rock. Watch for swelling or fuzzy patches that suggest infection.
- Cleanup crew disappearing: not always a “problem,” but don’t be shocked if small shrimp become an expensive snack.
If your Comet is breathing hard, hiding more than usual, and not coming out even for food, check oxygen and ammonia/nitrite immediately. They’ll tolerate a lot, but bad water hits them fast because they already like staying tucked away.
A simple routine that works: keep a consistent feeding schedule, keep one “no disturbance” cave zone, and don’t pair them with hyperactive food hogs. Do that and they turn into one of the coolest, most underrated marine fish you can keep.
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