Mojarita

Where they come from

Mojarita (Knodus breviceps) is a small South American characin. You see Knodus pop up from the Amazon and its tributaries and floodplain creeks - the kind of places with tea-colored water, leaf litter, tangles of roots, and calm edges where tiny fish can dodge bigger mouths.

In the tank, they act like a classic little river fish: always in motion, always hunting for specks of food in the water column.

Setting up their tank

These do best in a group, so think of them as a "schooling fish that needs space" more than a nano centerpiece. I'd start at 20 gallons long for a proper group, and bigger just makes life easier. They use the whole middle of the tank, and they appreciate length more than height.

• Tank size: 20 gallon long minimum for a group, 30+ is more relaxed
• Group size: 10-12+ if you can (they look and act better)
• Filtration: steady, not blasting - sponge prefilter helps with tiny mouths and fry
• Flow: moderate with calmer zones; they like to cruise, not fight a torrent
• Lights: medium; dimming it with floaters makes them bolder

I like a natural-ish setup: sand or fine gravel, some leaf litter if you enjoy the look, and lots of plants or wood to break up sight lines. They will still hang in the open, but they settle faster when they have edges to patrol and places to duck into.

Water-wise, they are pretty forgiving once settled, but they do not love sudden swings. If your tap is on the hard/alkaline side, they can adapt, just keep it stable and keep up with water changes. If you can offer softer, slightly acidic water, you will usually see better color and calmer behavior.

What to feed them

They are small-mouthed micro-predators and opportunists. In practice, that means they will eat most things you offer, as long as the particle size is right. Think "tiny bites, multiple types" rather than big pellets.

• Staple: quality micro pellets and/or fine flake (crushed)
• Frozen: cyclops, daphnia, baby brine shrimp, finely chopped bloodworms (not too much)
• Live (if you can): bbs, grindal worms, daphnia - this gets them really going
• Occasional: powdered fry food or crushed pellets when you want them to fill out

I feed small amounts 1-2 times a day. If you dump a big pinch in, the boldest fish will hog it and the rest will spend more time chasing than eating. Two smaller feedings keeps the group more even.

How they behave and who they get along with

Mojarita are lively, alert, and mostly peaceful, but they are still characins. In a small group they can get nippy and chasey, especially during feeding. In a bigger group they usually settle into that nice, busy schooling behavior.

• Best tankmates: other peaceful community fish that like similar water (small tetras, pencilfish, hatchetfish, corydoras, otos, calm dwarf cichlids)
• Use caution with: long-finned fish (they may test fins), very tiny shrimp, very tiny nano fish
• Avoid: aggressive cichlids, fin-biters, anything big enough to view them as snacks

They are fast and they spook easily if the tank is bare. Sudden shadows or loud movement can send them into "panic laps". That is normal, but if it happens all day, add more cover and check for bullying.

Breeding tips

They are egg scatterers. Spawning is possible in the home aquarium, but it is one of those projects where you get results by setting the stage, not by trying to "pair" them like cichlids.

If you want to try, use a separate breeding tank with a mature sponge filter, dim light, and either a thick mat of fine-leaved plants (java moss works) or a spawning mop. The goal is to let eggs fall where adults cannot easily pick them off.

• Group breeding works best: 6-10 fish with more females than males if you can
• Condition heavily: live/frozen foods for 1-2 weeks
• Trigger: slightly cooler water change, then bring it back up over a day or two
• Remove adults after you see spawning activity or after the morning rush

Fry are tiny at first. You will need infusoria, rotifers, or a very fine powdered food for the first stretch, then baby brine shrimp once they can take it. If you can keep the water super clean without swinging parameters, they grow steadily.

Common problems to watch for

Most issues I have seen with these come down to stress (too few fish, too bare a tank, too much chasing) or water getting dirty in an "invisible" way (overfeeding fine foods, not enough maintenance for an active school).

• Fin nipping and chasing: usually a small group or cramped tank; add more fish and more cover
• Skittish, constant dashing: bright lighting and no surface cover, or a bully in the tank
• Slow weight loss: food pieces too large, internal parasites in new imports, or dominant fish hogging meals
• Ich and other spotty outbreaks: often after a new addition or temperature swings
• Sudden deaths after purchase: shipping stress plus unstable water; they dislike big parameter jumps

If you see clamped fins, hiding, or they stop schooling and start hovering, I treat that as a "something is off" alarm. Check ammonia/nitrite, then look at temperature stability, oxygen (surface agitation), and whether you have one fish being a jerk. Fix the cause first, then reach for meds if you still need them.