Sulawesi Shrimp Died Overnight: What to Check This Morning

Why would a Sulawesi tank that looked fine at lights out hold dead shrimp by morning? Almost always because something changed in the previous 24 hours, in the water or in the room around it, and the readings that would name it start to fade the moment you begin fixing things. So test first and correct second, and the rest of this page walks you through both.

Sulawesi shrimp (lake Caridina) are kept at 79-86°F (26-30°C) and pH 7.5-8.5. That water is warm enough and alkaline enough that a small fault does more damage in one night than it would do in a week in a cool, slightly acidic tank. That’s the honest reason these losses tend to arrive all at once, and it’s why the first hour this morning matters more than the next three days.

What to do in the first ten minutes after a Sulawesi overnight loss

Do the reversible things now and nothing else. For the next ten minutes your job is to stop the tank getting worse without wiping out the evidence, because the water in front of you is the only record of what happened while you slept.

1. Take the readings before anything else. Ammonia, nitrite, temperature, pH, KH, GH and TDS, from water drawn before you disturb the tank. Ten minutes with a test kit beats a day of guessing.
2. Lift the bodies out. Left in, they add organic waste to a tank that may already be carrying too much.
3. Increase surface movement, gently. Raise the filter outlet so it breaks the surface, or run an air stone on a low setting. It’s the one change worth making before you know the cause.
4. Check the heater reading against a second thermometer. Not the heater’s own dial. Use an independent probe or a glass thermometer in the tank.
5. Write down what you did to the tank yesterday. Feeding, a water change, a top-off, a new rock, a new plant, a filter clean, anything sprayed or plugged in nearby.

Get oxygen in without pushing the survivors around

Warm water holds less dissolved oxygen than cool water.¹ At Sulawesi temperatures you’re already working with a thinner reserve than a Neocaridina keeper has, so the safest thing you can do this morning is improve gas exchange without turning the tank into a current.

You don’t need the same equipment, just the same idea. Aim the return so it ripples the surface instead of driving down into the substrate. If you add an air stone, start it low and watch where the shrimp go. Survivors that leave the rock and cling to the glass near the outlet are telling you the flow is too much for them right now. Keeping the tank near saturation from here on is a setup question, not an emergency one, and it’s covered in Sulawesi shrimp oxygen requirements.

Did your Sulawesi colony go in one night, or has this been building?

The shape of the loss narrows the cause list faster than any single test. A whole colony gone between lights out and breakfast points at something that arrived. Losses that add up over a fortnight point at something that drifted.

So count before you pick a side. How many are dead, how many are still alive, and how many did you lose in the two weeks before this morning? If you keep a log, check it now. If you don’t, this is the morning to start one. A tank that had been losing a shrimp every few days and then lost a dozen overnight needs both lists checked, because something may have pushed it over after something else had been wearing it down.

Two other patterns get mistaken for this one. Did you do a water change yesterday evening? That timing changes the whole cause list, and Sulawesi shrimp dying after a water change works through it. And if the shrimp arrived in the last week, you’re probably looking at the move and the tank they moved into, not at an overnight event.

What the bodies tell you before you bin them

Look at them properly once, in good light, before they go. You aren’t diagnosing anything yet. You’re sorting one pattern from another.

• Bodies gathered high in the tank, on the heater top, the filter rim or right at the waterline, with no other obvious damage. Note that against your oxygen and temperature readings.
• Juveniles and the smallest shrimp gone first, with adults following. Write down the order, because it’s information even when it doesn’t name a cause.
• Survivors twitching, lying on their sides, or swimming in a way you haven’t seen before. The living tell you more than the dead do.
• A white gap at the carapace joint with a half shed shell beside it. That shrimp died in a molt, not from whatever hit the colony, and Sulawesi shrimp stuck in a molt covers what to check next.
• Opaque, milky muscle through the tail. That sign is described in a virus disease of a farmed freshwater prawn, a different animal, so keep it as a separate pattern.²

Which Sulawesi readings to take first, while the evidence is still there

Test in this order, because the early readings change what the later ones mean: ammonia and nitrite first, then temperature, then pH and KH, then GH and TDS. Compare what you get with the numbers below.

A clean reading this morning doesn’t clear the night. Ammonia from a heavy evening feed can be eaten by the filter before you test. A heater that ran hot for six hours can be back on target by nine. That’s why your notes about yesterday matter as much as the test kit.

Why the same ammonia reading is worse at pH 8.5

Total ammonia in water splits between the ammonium ion and un-ionized ammonia, and the un-ionized form is the one that drives toxicity. The split depends on pH and temperature. The ratio of un-ionized ammonia to ammonium rises about tenfold for each single pH unit, and roughly doubles for each rise of 10°C between 0 and 30°C.³

Run those numbers and the result is blunt. A 0.25 ppm total ammonia reading at pH 8.5 carries on the order of a hundred times the un-ionized share that the same reading would carry in a bee shrimp tank at pH 6.5, and that’s before the warm water adds anything. That comparison is worked out from the published relationship, not measured in Caridina, but the direction isn’t in doubt. A number a bee keeper would shrug at isn’t one you can shrug at.

High external un-ionized ammonia reduces or reverses the gradient the animal uses to shed its own ammonia, so it builds up in tissues and blood instead of leaving.³ An animal in that state also extracts oxygen less well. That’s where the two problems in this section stop being separate problems.

Oxygen falls all night and bottoms out near dawn

Nothing photosynthesizes in the dark. Plants and algae are usually the biggest oxygen producers in a system, and they’re also among its biggest users, while decomposing organic matter uses oxygen too. The result is a daily cycle: oxygen climbs through the lit hours, falls through the night and hits its lowest point before sunrise.⁴

Now put that cycle in a tank held at 79-86°F (26-30°C), where the water already holds less oxygen to begin with, and add a heavy feed that gives the bacteria something to work on overnight. The bottom of that curve arrives while you’re asleep, just before you find the bodies. A tank that’s been fine for months can reach it for the first time after one warm night, one extra portion of food, or one filter that quietly stopped.

So check that the filter is actually moving water and not just humming, and that nothing has slid over the outlet. Did the room get warmer overnight, say because the air conditioning went off? That alone lowers what the water can hold while raising what the animals need.

Clean Sulawesi readings point away from the water and into the room

This is the part most cause lists skip. If ammonia, nitrite, temperature, pH, KH and TDS all come back where they should be and the colony is still dead, the water probably wasn’t the problem. Something got into it, or the room did something to it.

Before you look at the tank again, retrace the room as it was last night. Was anything sprayed, plugged in or switched on in that room, or in one sharing its air? Did anyone put a hand, a net or a bucket in the water, and what had it touched before? Did the room get unusually hot or cold? Write the answers beside your readings. Each of the three causes below can leave a tank testing clean by morning while the colony is gone, which is exactly why they get missed.

Mosquito vaporizers, coils and sprays running overnight

Shrimp are arthropods, and the products sold to kill arthropods in your bedroom don’t tell the difference. Permethrin, one of the pyrethroid insecticides, works by interfering with sodium channels in nerves, which makes muscles spasm and ends in paralysis. It’s described as highly toxic to freshwater fish and other aquatic organisms, and highly toxic to invertebrates.⁵ Household mosquito products use related pyrethroids. What we know is the chemical family and how it acts, not a dose for your tank.

The living shrimp are your evidence here. Uncoordinated swimming, twitching pleopods and animals lying over on their sides, in a tank where every reading is normal, is the pattern to take seriously. If you suspect it, move the air pump intake to another room or outside a window, and run fresh activated carbon in the filter.

A heater that stuck on or stuck off overnight

A heater that fails closed keeps heating a tank it should have stopped heating. A heater that fails open lets a warm alkaline tank drop out of range while you sleep. Neither one leaves a trace in a chemistry test by morning, which is why the temperature you read at 7am can look perfect and mean nothing.

Feel the heater, look for condensation or a cracked tube, and if you have any doubt, unplug it and put in a spare. A cheap thermometer that logs a minimum and maximum is the least expensive insurance in this hobby. It turns the next night from a guess into a record.

Copper that came in with the water or with a product

Elevated copper is toxic in aquatic environments and can affect invertebrates as well as fish, with acute effects that include mortality. How toxic it is depends on water chemistry, including pH, alkalinity, dissolved organic carbon and hardness.⁶ In a juvenile freshwater prawn, copper damaged the gills and the hepatopancreas within a week.⁷ No safe or lethal copper figure has been established for Caridina, so treat copper as something to exclude, not something to measure against a threshold.

The usual ways it gets in are ordinary. Tap water that ran through copper plumbing. A plant fertilizer containing chelated copper. A fish medication used in the same tank, or with the same bucket. A snail treatment. Did you top off or change water with anything other than your usual prepared water last night? Then that’s your candidate.

Correct a Sulawesi tank slowly enough to keep the survivors

Here it is plainly: the correction kills more Sulawesi colonies than the fault does. The shrimp still alive this morning have already taken one hit, and they’re the ones who’ll pay for a fast fix.

• Replacing a large share of the water at once. If you must change water because ammonia is present, go small, and match temperature, pH, KH, GH and TDS to the tank before it goes in.
• Adding water straight from the tap, or from a fresh mineral mix you haven’t tested. Mix and measure outside the tank, every time, emergencies included.
• Chasing pH with a product. Sudden pH changes in a stressed tank are a worse problem than a pH sitting slightly off.
• Dosing a medication at a colony that hasn’t been diagnosed. There’s nothing to treat if the cause was oxygen, temperature or an aerosol, and the treatment is its own stressor.
• Feeding as normal tonight. A smaller colony eats less, and the leftovers feed the next ammonia rise.
• Tearing down the hardscape to find the culprit. Disturbing the biofilm and the filter bed strips out the processing capacity the tank has left.

The things worth doing are dull. Hold the Sulawesi shrimp tank temperature steady at one point in the range. Keep the surface moving. Feed lightly or not at all for a couple of days, and lift anything uneaten within a few minutes. Test daily for a week so you see a trend instead of a snapshot. If a reading needs to move, move it over days, in steps small enough that the shrimp don’t register the change.

When a Sulawesi overnight loss has gone past what testing can answer

Some of these nights never get explained, and it’s more useful to say so than to pick a cause and act on it. There’s no published tolerance figure for ammonia, oxygen or copper for this group, and the signs you can see after the fact don’t map cleanly onto causes. What you can do is exclude what’s testable and remove what’s removable.

Get qualified help when losses continue after the water tests clean and the room has been cleared, when survivors show signs you can’t account for, or when you’re thinking about any treatment. An aquatic veterinarian or a specialist who can examine the animals is the right next step, and a care guide isn’t a substitute for that. If you’d like another pair of eyes on the numbers first, send your ammonia, nitrite, nitrate, pH, KH, GH, TDS, temperature, tank size, cycle status and current livestock to contact@shrimpaquatics.com and we’ll tell you honestly what we’d check next.

What the research says about overnight Sulawesi shrimp deaths

Treat the milky tail sign and the copper warning on this page as things to rule out in your shrimp, not as measurements of them. Both come from work on the giant freshwater prawn, Macrobrachium rosenbergii, a farmed species and not a lake Caridina. Opaque abdominal muscle is described in white tail disease, a virus disease of that prawn, where it comes with very heavy losses.² It’s useful as a pattern to keep apart from the others, not as a diagnosis of your shrimp.

The copper work is on the same prawn. In a laboratory study on juveniles, waterborne copper from 0.01 mg/L to 0.4 mg/L over seven days caused structural damage to the gills and the hepatopancreas, and the damage was worse at the higher concentrations.⁷ It’s a different animal in different water, which is why this page asks you to rule copper out and doesn’t offer a number to test against.

References

7 Sources

1. Dissolved Oxygen and Water, U.S. Geological Survey Water Science School, last updated August 24, 2026.
2. White Tail Disease of Freshwater Prawn, Macrobrachium rosenbergii (abstract), Indian Journal of Virology 23(2), Springer, 2012.
3. Aquatic Life Ambient Water Quality Criteria for Ammonia, Freshwater, U.S. Environmental Protection Agency Office of Water, April 2013.
4. Dissolved Oxygen for Fish Production, FA002, Ruth Francis-Floyd, University of Florida IFAS Extension, April 2021.
5. Permethrin Technical Fact Sheet, National Pesticide Information Center, Oregon State University Extension Services, March 2009.
6. Supplementary Training Materials for Aquatic Life Criteria, Copper, Background, U.S. Environmental Protection Agency, last updated May 6, 2026.
7. Impact of waterborne copper on the structure of gills and hepatopancreas and its impact on the content of metallothionein in juvenile giant freshwater prawn Macrobrachium rosenbergii (abstract), Archives of Environmental Contamination and Toxicology 52(1), Springer, 2007.