Corals not opening after a water change: what to do
You did a water change a few hours ago. The euphyllia that was fully inflated this morning is a shriveled skeleton, the zoas are shut, and the SPS have no polyp extension at all. Nothing looks dead, but nothing looks right either.
The reassuring part first: this is one of the most common things in the hobby, and the overwhelming majority of the time the corals are back to normal by the next day. Corals close as a reflex. It is a response to change, not proof of damage. What matters is how long they stay closed and whether anything else is happening besides closure.
What to check right now
Do these in order, before you change anything else.
- Temperature. Put a thermometer in the display. Compare it to what the tank read before the change. If it moved more than 1°F (0.6°C), you found a contributor.
- Salinity. Measure with a refractometer or calibrated conductivity meter, not a swing arm hydrometer. You want 1.025 to 1.026 specific gravity (34 to 35 ppt).
- Alkalinity. Test the tank now. If you know what it was before the change, compare. This is the single most likely chemical cause.
- Look for tissue, not behavior. Closed polyps are fine. Receding tissue, exposed white skeleton at the base, stringy sloughing mucus, or brown jelly is a different and more urgent problem.
- Check the obvious mechanical stuff. Did a powerhead get bumped into a new angle? Is a return nozzle now blasting something that used to sit in medium flow? Is detritus settled on top of a coral?
- Do nothing else. Resist the urge to dose, add carbon, or do a second water change to "fix" it. A second change on top of the first is how a small swing becomes a big one.
How long is normal, by coral type
These are typical recovery windows for a healthy coral after a routine water change.
| Coral type | Usually reopens within | Worry after |
|---|---|---|
| Zoanthids and palythoa | 1 to 6 hours | 48 hours |
| LPS (euphyllia, acans, blastos, favia) | 2 to 12 hours, often by the next lights on | 48 hours |
| SPS (acropora, montipora, stylophora) | 4 to 24 hours for polyp extension to return | 48 hours, or any color change |
| Soft corals (leathers, sinularia) | 12 to 48 hours | 5 days |
| Mushrooms and ricordea | 1 to 12 hours | 48 hours |
Leathers deserve a note. They routinely close for two to five days while shedding a waxy surface coat, and it often happens right after a water change because the change triggered the shed. A closed, slightly slimy leather that is otherwise firm and not receding is almost always fine.
The general rule: 24 hours closed is common, 48 hours closed means something is still wrong right now, and it is worth finding rather than waiting out. Beyond that, if you are seeing tissue recession or color loss instead of simple closure, stop thinking of it as a water change reaction. That is a chemistry problem that is still ongoing.
What actually caused it, ranked
In rough order of how often each one is the real answer.
1. Temperature mismatch. The most common cause by a wide margin, and the easiest to fix. New water sitting in a garage or basement can be 5 to 10°F below tank temperature. Swapping 20% of the volume with water that is 8°F cold drops the display by roughly 1.6°F in minutes. Corals notice a fast 2°F swing immediately.
2. Salinity swing. Mixed water that came out at 1.028 instead of 1.025 will pull the whole tank up. Top off evaporation before you take water out, not after, otherwise you are measuring and matching against a concentrated tank.
3. Alkalinity swing from the new water. This is the one that catches experienced reefers, because everything else was done right. Salt mixes vary enormously out of the bucket:
- Tropic Marin Pro Reef: about 7.5 to 8.0 dKH
- Red Sea Blue Bucket: about 8.0 dKH
- Instant Ocean: about 10 to 11 dKH
- Reef Crystals: about 11 to 12 dKH
- Red Sea Coral Pro: about 11 to 12 dKH
If your tank runs 8.0 dKH and you switch to a mix that lands at 12.0, a 25% change puts you at 9.0 dKH in one afternoon. That is a 1.0 dKH jump, which is past the threshold where SPS visibly sulk and, if repeated, past where tips start to burn.
The math is simple enough to do in your head:
New tank alk = (tank alk × (1 - change fraction)) + (new water alk × change fraction)
Run it before you pour, not after.
4. An under-mixed or freshly mixed salt batch. Salt that has been circulating for 20 minutes is not finished. Undissolved carbonates keep dissolving for hours, so a batch tested at hour one reads differently than the same batch at hour twenty. Worse, mixing salt into water that is already hot, or aerating hard while it is still cloudy, can precipitate calcium carbonate out of solution and leave you with mixed water that is genuinely low in alkalinity and calcium.
5. pH difference. Fresh RODI is CO2 saturated and acidic. Newly mixed saltwater often sits at 7.8 to 8.0 and climbs to 8.2 to 8.4 after a day of circulation. A 0.2 unit dip in the display is usually not enough to close corals on its own, but it stacks with everything above.
6. Flow change and stirred detritus. Siphoning the sand bed releases a cloud of organics that settles on coral tissue. Corals shut down under a detritus dusting and reopen once flow clears it. A powerhead moved during cleaning and left pointed 20 degrees off is a common and easily missed cause.
7. You bumped it. Hands and siphon hoses in the tank. A hammer coral brushed with a hose stays deflated for the rest of the day. Completely normal.
8. A new bucket of the same salt. Batch to batch variation within a single brand is real, typically half a dKH but occasionally more. Treat every new bucket as an unknown until you have tested a mixed batch from it.
The diagnostic that beats guessing
Here is the thing forum threads never tell you: you cannot diagnose this from memory. "It was around 8, I think" is not data. You need the before number and the after number for the same parameter, taken the same way with the same test kit.
At minimum, log these before a water change and again two hours after:
| Parameter | Fine | Suspect | Almost certainly the cause |
|---|---|---|---|
| Temperature | under 0.5°F change | 0.5 to 1°F | over 1°F (0.6°C) |
| Salinity | under 0.001 sg | 0.001 to 0.002 sg | over 0.002 sg (2.5 ppt) |
| Alkalinity | under 0.3 dKH | 0.3 to 0.7 dKH | over 0.8 dKH |
| pH | under 0.1 units | 0.1 to 0.2 units | over 0.3 units |
| Calcium / magnesium | any normal reading | rarely relevant acutely | almost never the acute cause |
Alkalinity is at the top of the list for a reason. It is the parameter that moves hardest with a salt change, and it is the parameter corals are least tolerant of moving fast. Corals handle 7.5 dKH fine and 10 dKH fine. They handle 8 to 10 in one afternoon badly.
Also test your mixed water, not just the tank. If you only ever test the display, a bad batch is invisible until it is already in the system. And if you are not sure what your tank's alkalinity normally does day to day, that baseline is worth establishing first, because it changes how you read every one of these numbers. See daily alkalinity consumption for how to measure your own drawdown rate.
How to do water changes so this stops happening
None of this is difficult. It is just a routine.
- Mix and circulate for 24 hours with a powerhead before use. Not 30 minutes, not 2 hours. Salts with high alkalinity targets need the full day to finish dissolving.
- Mix at room temperature, then heat. Adding salt to hot water invites precipitation.
- Match temperature to within 1°F. A cheap 50W heater in the mixing bucket, turned on the night before, solves this permanently.
- Match salinity to within 0.001 sg. Calibrate the refractometer with 35 ppt calibration fluid, not RODI, and recalibrate every couple of months.
- Test the mixed water before it goes in. Alkalinity at minimum. Thirty seconds of work that catches the worst failure mode.
- Change 10 to 20% at a time, weekly or biweekly. A 20% change caps how far any single parameter can move. If you feel you need a 50% change, do two 20% changes a few days apart instead.
- Top off evaporation first, so the tank you are matching against is at its real salinity.
Do those seven things and the closure problem mostly disappears.
When it is not the water change at all
The water change is a convenient thing to blame because you were standing there when it happened. Sometimes it is a coincidence.
- Alkalinity was already drifting. If your dosing has been falling behind for a week, the corals were already stressed and the change was the last straw. The fix is the dosing schedule, not the water change routine.
- Photoperiod. Many LPS and most zoas close on their own schedule. If you changed water in the evening, you may just be watching a normal night cycle.
- Pests. Flatworms, montipora nudibranchs, red bugs, sundial snails, and aggressive aiptasia all cause closure that gets misattributed. Look at the coral with a flashlight after lights out.
- Neighbor aggression. A coral that grew enough to reach a neighbor's sweeper tentacles closes on one side only. Directional closure is a strong hint.
- Your RODI. Spent DI resin can push silicates, ammonia, or copper into your mixed water while the tank looks fine. Check TDS out of the unit reads 0.
- Contamination on your hands. Lotion, soap residue, or a mixing bucket that was used for something else once.
The tell for all of these is that the corals do not recover on the normal timeline. A water change reaction fades. An ongoing problem does not.
The short version
Closed corals a few hours after a water change is normal and usually resolves the same day. Past 48 hours, or with any tissue recession, it is not a reaction and you should be looking for a cause. That cause is temperature, salinity, or alkalinity roughly nine times out of ten, in that order of frequency, and you find it by comparing numbers from before and after, not by remembering.
Recif is the reef app we are building for exactly this problem. It logs every test, charts each parameter against its target band so a swing is obvious at a glance, and flags the drop before your corals do. It is still in development, and you can see what we are working on at our apps page.