Why Your Sulawesi Shrimp Are Swimming Erratically

Your Sulawesi shrimp have left the rock and are swimming laps around the tank. That’s one of two things: adult males searching for a female that has just molted, or a fault in the water. Who is swimming tells you which, and you can usually see it in under a minute. Males alone means wait it out. The whole colony off the rock means test the water now.

Who is swimming tells you whether your Sulawesi shrimp are mating or in trouble

If only some adult males are swimming, they’re almost certainly mating, and the burst passes on its own. Look for a freshly cast shell first, because a female becomes receptive right after she molts, and that’s when the mating signs in Sulawesi shrimp start. Research on other shrimp ties mating to the female’s fresh molt and finds it’s over within seconds or minutes.¹²

None of that was measured in Sulawesi shrimp, so you’re matching a pattern, not a number. A handful of adult males quarter the glass and swim into the filter current, while the females stay out of sight in the rockwork. It usually settles within a few hours. There’s nothing to fix, so put the test kit down and let them get on with it.

What to test first when the whole Sulawesi colony is off the rock

Test before you adjust anything. Run these in order, because the first two tell you whether it’s an emergency and the rest tell you what moved.

• Ammonia, with a liquid reagent kit, not a strip. It should read zero.
• Nitrite. It should read zero too. A tank that can’t hold both at zero isn’t ready for these shrimp.
• Temperature, at the glass and again at the far end. A stuck heater shows up here before it shows up anywhere else.
• TDS, then pH. You’re looking for movement since your last reading, not for a perfect number.
• The surface. Is it moving at all, and is anything sitting on the filter outlet?

Why warm alkaline water makes the same ammonia reading worse

Because more of the ammonia is in its toxic form. Total ammonia splits between the ammonium ion and un-ionized ammonia, and it’s the un-ionized form that does the damage. The EPA’s freshwater ammonia criteria put it plainly. The ratio of un-ionized ammonia to ammonium ion in fresh water rises 10-fold for each single pH unit, and roughly two-fold for each 10 degrees Celsius of warming from 0-30 degrees Celsius.³ At high outside concentrations the un-ionized form reduces or reverses the animal’s diffusive gradients, so ammonia builds up in its tissues and blood.³

That document covers a whole freshwater community, not shrimp, and it sets no aquarium threshold. What it gives you is the direction, and the direction is the point. A Sulawesi tank runs at pH 7.5-8.5 and 79-86°F (26-30°C). A Bee tank runs a couple of pH units lower and much cooler. So the same reading on the test card means a very different amount of free ammonia in those two tanks. An uneaten pellet you’d get away with in soft acidic water is worth taking seriously here.

Why shrimp collect around the filter outlet when the tank runs warm

They’re looking for oxygen. Cold water can hold more dissolved oxygen than warm water.⁴ USGS is describing surface water there, not aquariums, but a tank held in the high 80s Fahrenheit still has less room for oxygen than the same tank in the low 80s. A flat, still surface takes away the rest. If your shrimp are hanging on the outflow or riding the surface film, fix oxygen first. It’s also the only fix on this page that can’t shock them.

Retrace what went into the Sulawesi tank in the last two days

Colony-wide swimming that starts suddenly almost always follows something you did. Retrace the last couple of days honestly, including the thing you’re sure was fine.

Write it down with rough times: every top-off and water change, any mineral batch, a new plant, rock or decoration, anything dosed, and any filter cleaning. Then put it next to your test results. A TDS that has climbed points at top-offs. A pH or KH that moved points at something mixed or dosed. Clean readings plus something new in the tank point at contamination a standard kit won’t show, such as copper.

Mineral salt or powder that went in before it had dissolved

Sulawesi Mineral 7.5 and 8.5 don’t mix the same way. SaltyShrimp’s own instructions for Sulawesi Mineral 8.5 say to add it to water outside the aquarium. The salt dissolves almost completely only after CO2 is run into it for about 3-4 days at roughly one bubble a second. The water then needs at least three hours of air stone aeration to drive that CO2 off before use.⁵ Tip it into a display tank early and you’ve added undissolved solids and a gas swing to a colony that was fine an hour ago. The same goes for bacterial and enzyme powders. Mix everything outside the tank, and give it time.

Why topping off with remineralized water pushes TDS up

Water leaves your tank as vapor, and the minerals stay behind. Evaporation concentrates dissolved solids in the water that’s left.⁶ USGS describes that for groundwater, not aquariums, but your TDS meter does the same arithmetic. Top off with plain RO water and save the remineralized water for real water changes. If your TDS has climbed since last week and nothing else has changed, this is usually why.

Copper, new tap water and fish medications a shrimp colony cannot take

Elevated copper is toxic in aquatic environments and may harm invertebrates. How toxic it is depends mostly on how much of the metal binds to a biochemical receptor such as a gill membrane, and pH, alkalinity, dissolved organic carbon and hardness all change how available it is.⁷ In practice, suspect new tap water, a plant from an unknown system, or a fish medication dosed into a shared tank. Should you reach for a fish treatment while you’re still guessing? No. A stressed invertebrate colony is the worst possible place to test a formulation.

How to steady a Sulawesi tank without causing a second shock

1. Add air. An extra air stone costs nothing and carries no risk. Aim it for surface movement, not a current through the shrimp.
2. Stop feeding. Lift anything uneaten within a few minutes. In warm alkaline water, leftovers are the fastest route back to the ammonia problem you just tested for.
3. Prepare replacement water outside the tank to stabilize Sulawesi shrimp water. Mix it, aerate it, warm it until the thermometer matches the display, and test the finished water before any of it goes in.
4. Drip it in. Keep the exchange small. A tenth of the tank, dripped at one to two drops a second, does more for a stressed colony than a third poured in.
5. Then wait. Retest the next morning and the morning after. Don’t change a second thing before you know what the first one did.

When one Sulawesi shrimp is swimming badly and the colony is fine

Then it’s usually not the water. A single animal acting strangely while everyone else grazes is a different problem. A Sulawesi shrimp with a failed molt will thrash, snap its tail and dart backwards. There’s nothing you can safely add to the tank to help it in that moment. Leave it alone, keep the water steady, and let the colony be your evidence: if nobody else follows, your parameters probably aren’t the cause.

Tissue that has gone milky or opaque is a separate thing from a shrimp that’s simply pale. In farmed freshwater prawns, opaque tail muscle is a described sign of a disease that can kill fast.⁸ It’s a different species, so it isn’t a diagnosis for your tank, and nobody has published what opaque muscle means in lake Caridina. If you see it spreading through a colony, ask a qualified aquatic veterinarian before you dose. Telling those two apart before you treat anything is the first step in common health problems in Sulawesi shrimp.

What the research says about erratic swimming in Sulawesi shrimp

The mating and disease studies behind this guide are on other animals, so read them as the mechanism, not as a measurement of your tank. In freshwater prawns, successful mating happens only between a female that has just finished her pre-mating molt and is still soft-shelled, and hard-shelled males.¹ In shrimps where males search for females instead of guarding them, recognition and mating happen within seconds or a few minutes, and the partners separate quickly.² That short window of attractiveness is what spares the female harassment serious enough to injure or kill her.² It matches the pattern above: a burst of searching by the males that settles within hours.

The disease side is starker. In farmed giant freshwater prawns, opaque abdominal muscle alongside lethargy and degeneration of the telson and uropods is a described clinical sign of white tail disease. In the animals studied, it caused up to 100 per cent mortality within four days.⁸ No equivalent work exists for Sulawesi shrimp, which is exactly why a vet is the right call when opaque tissue spreads.

References

8 Sources

1. Farming freshwater prawns: a manual for the culture of the giant river prawn (Macrobrachium rosenbergii), biology chapter, Food and Agriculture Organization of the United Nations, FAO Fisheries Technical Paper 428.
2. Ray Bauer research: mating behavior and sexual strategies, Raymond T. Bauer, University of Louisiana at Lafayette, research page.
3. Aquatic Life Ambient Water Quality Criteria for Ammonia, Freshwater, U.S. Environmental Protection Agency, Office of Water, April 2013.
4. Dissolved Oxygen and Water, U.S. Geological Survey, Water Science School, updated August 24, 2026.
5. Sulawesi Mineral 8,5: minerals and trace elements, SaltyShrimp, manufacturer product information.
6. Chloride, Salinity, and Dissolved Solids, U.S. Geological Survey, Water Resources Mission Area, March 1, 2019.
7. Supplementary Training Materials for Aquatic Life Criteria, Copper, Background, U.S. Environmental Protection Agency, updated May 6, 2026.
8. White Tail Disease of Freshwater Prawn, Macrobrachium rosenbergii, abstract, Indian Journal of Virology 23(2), 2012.