Mary River cod

Where they come from

Mary River cod are an Australian native from the Mary River system in southeast Queensland. They are a river predator that likes structure, current breaks, and clean water. In the wild they have had a rough run, so any fish you see in the hobby should be captive-bred and legally sourced.

Setting up their tank

This is one of those fish where the tank is the easy part and the total system is the hard part. They get big, they are powerful, and they make a mess when they eat. Think less "aquarium" and more "indoor pond with a viewing panel" if you plan to keep one long-term.

• Tank size: for juveniles you can start big (a few hundred liters), but adults quickly outgrow typical home tanks. Plan for a very large footprint and depth, with room to turn easily.
• Filtration: oversize everything. Big canister + sump, or a proper pond-style filtration setup. You want lots of mechanical removal and a ton of bio media.
• Flow and oxygen: moderate flow with calmer zones. Strong aeration is your friend, especially in warm weather.
• Temperature: they handle a range, but stability matters more than chasing a number. Avoid sudden swings.
• Substrate: sand or smooth gravel is easiest. They will bulldoze decor, so skip anything sharp.
• Cover and structure: big driftwood, rock piles that cannot shift, and cave-like retreats. Give them at least one "home" they can claim.
• Lid: tight-fitting. Cod can spook and launch, especially at night or during maintenance.

I like to build the layout around one or two main lairs and keep the rest open for cruising. If you stack rock, use egg crate or a solid base under it, and silicone or cable-tie where you can. Driftwood should be heavy enough that it cannot roll if the fish rams it.

What to feed them

They are ambush predators. They will take pellets, which is the best thing you can train early, but they also love meaty foods. The trick is keeping them in good shape without turning the tank into a grease trap.

• Staple: high-quality sinking predator pellets (train this while they are small).
• Good supplements: prawn/shrimp, mussel, squid, white fish fillet, earthworms, insects.
• Occasional treats: whole freshwater cray (if legal), small chunks of lean meat.
• Avoid: feeder fish. They bring parasites and often lead to fatty, vitamin-poor diets.
• Avoid: too much oily fish (like salmon) - it can foul water and pack on fat quickly.

Feeding schedule depends on size. Little ones can eat small meals most days. Bigger fish do better with fewer, larger meals. If the belly is staying rounded for days, you are feeding too heavy.

How they behave and who they get along with

Mary River cod are confident, territorial predators. Some are chill, some are absolute jerks, and they can switch moods as they grow. You will get the classic cod behavior: sitting in a hide, watching you, then exploding into motion at feeding time.

• Temperament: territorial, especially around their chosen cave/log.
• Tankmates: safest plan is species-only. If you try tankmates, they must be large, robust, and not shaped like food.
• Risk zones: anything that fits in their mouth is food, and anything that competes for the same space may get smashed.
• With other cod: only in very large systems, and even then expect aggression as they mature.

If you insist on tankmates, build lots of line-of-sight breaks and multiple heavy hides. Feed in a way that spreads food out so one fish does not guard the whole meal. And have a backup plan and spare tank ready, because you may need to separate fast.

Breeding tips

Most hobbyists will not breed this species at home. They are big, seasonal, and pairing them safely takes space and planning. In Australia, breeding can also be regulated, so treat this as background rather than a weekend project.

• They are cavity spawners in nature, using hollow logs/undercut banks.
• Spawning is linked to seasonal cues (temperature and flow changes).
• Males typically guard eggs and the nesting site.
• If you ever attempt it, you are looking at a pond-scale setup, conditioned adults, and a plan for thousands of fry.

Common problems to watch for

Most issues I have seen with big cod-type fish come down to water quality, injuries from decor, and diet. They are tough, but they do not tolerate chronic dirty water.

• Spikes in ammonia/nitrite: usually after a big feeding or filter disruption. These fish can overwhelm a marginal filter fast.
• High nitrate and dissolved organics: shows up as lethargy, poor appetite, and generally "off" behavior.
• Mouth injuries: from smashing into glass/decor during feeding or spooking.
• Fin damage and scrapes: from squeezing into tight hides or rough rock.
• Hole-in-the-head style pitting: can appear in large predators kept in dirty water or with poor diet variety.
• Parasites: more common if you use feeder fish or wild-caught food.

Watch their posture and breathing. A cod that is hovering in the open, clamping fins, or breathing hard is telling you something. Test the water before you guess, and do not underestimate how much one big feeding can swing your numbers in a closed system.