Silver moony (Mono)

1) Where they come from (quick backstory)

Silver moonies (Monodactylus argenteus) are classic “coastal” fish. In the wild they cruise around estuaries, mangroves, and river mouths where fresh and salt water mix. That’s why they’re so unfazed by changing conditions… as long as you keep things clean and stable in your tank.

2) Setting up their tank

Think “open water with strong filtration.” Moonies are like little silver dinner plates that zip back and forth all day. If you cram them in a short tank, you’ll watch them bonk around and sulk.

• Tank size: I’d treat 4–6 juveniles as a 75g/280L minimum, and bigger is honestly nicer once they start putting on size.
• Footprint matters more than height: longer tanks help a lot.
• Flow + oxygen: they like brisk water movement and high aeration.
• Filtration: oversize it. They eat well and they poop like it’s their job.

Brackish-wise, I’ve had the best results raising them in lower-end brackish as youngsters and gradually bumping salinity as they grow. You don’t need to swing it around—just pick a target and keep it consistent.

Decor is simple: sand, a few pieces of driftwood/rock, maybe some hardy brackish-friendly plants if your salinity isn’t too high. But don’t turn it into a maze—leave them open swimming lanes.

3) What to feed them

These are hungry, curious eaters. Mine acted like piranhas at feeding time (without the bite marks). If you keep them well-fed, they’re calmer and less likely to pick at tankmates.

• Staples: quality pellets/flakes that are meant for omnivores/marine fish
• Frozen: mysis, brine shrimp (enriched if you can), krill chopped up, prawn, clam
• Greens: spirulina flakes, blanched spinach, nori sheets clipped to the glass
• Occasional treats: chopped earthworms or insect-based foods (rinse well)

They learn routines fast. If you hand-feed in the same spot, they’ll pile up there like a school of silver coins.

4) Behavior + tankmates

Moonies are schooling fish, and you can tell when they’re not happy: lone or paired moonies often turn jumpy and skittish, and they may start acting bossy. In a group, they spread the attitude out and settle into that smooth, coordinated cruising they’re known for.

• Keep them in a group (I like 5+ if your tank can handle it).
• Expect a pecking order—lots of chasing, usually not much damage if they’ve got space.
• They’re fast feeders, so slower fish can starve if you don’t target-feed.

Tankmates should like brackish water and be able to handle a busy, pushy feeding time. Good matches are other brackish “community” fish with some speed and confidence.

• Often works: scats (similar vibe), bumblebee gobies (if not outcompeted), knight gobies, monos of the same species, some brackish-tolerant puffers with caution (depends on the puffer)
• Use caution: slow, long-finned fish; tiny fish they can bully at meals
• Avoid: anything that needs pure freshwater long-term, or fish that nip fins constantly

5) Breeding tips (realistic expectations)

Breeding silver moonies in home aquariums is uncommon. Most people keep them as display/behavior fish rather than a breeding project. They’re believed to spawn in more marine conditions, and raising the larvae is the hard part.

6) Common problems to watch for

Most moony problems trace back to two things: cramped tanks and dirty water. They’re tough fish, but they don’t hide stress well—when they’re off, you’ll notice fast.

• Skin/fin issues (bacterial/fungal looking fuzz or frayed fins): often follows ammonia/nitrite spikes or chronically high nitrate
• Ich/white spot: can happen, especially after shipping; brackish setups sometimes make treatment choices tricky
• Hollow bellies/slow growth: usually from being outcompeted early on or being fed “light” because you’re afraid of waste
• Jumping/spooking: new fish, bright lights, sudden movement, or no cover
• “Pacing” and constant chasing: group too small or tank too tight

My personal rule with moonies: if they suddenly stop being bold at feeding time, I test water immediately. They’re usually the first fish in the tank to tell you something’s off.