Your nitrate of 80 ppm is the most urgent problem.

Critical Issues

Your nitrate of 80 ppm is the most urgent problem, and the timing makes it the clear culprit for both the closed hammers and the labored breathing in your tang. A large water change performed carelessly can also introduce a sudden shift in temperature, salinity, or pH, so if your replacement water was not matched precisely, that compounds the stress. Hard breathing in a fish is a serious warning sign of either chemical irritation or gill hypoxia. Watch that tang closely over the next few hours.

Your alkalinity of 7.1 dKH is below the stable reef range of 8.0-9.5 dKH. This alone can cause Euphyllia to close and polyp tissue to recede, so you may be dealing with two separate stressors hitting simultaneously.

Check whether your new saltwater was temperature- and salinity-matched before the change. A sudden osmotic or thermal shock on top of high nitrates would explain why symptoms appeared immediately after the water change rather than gradually.

Secondary Concerns

• pH at 8.1 is acceptable, but if your house CO2 rises at night the actual low point may be pulling alkalinity effects lower than your reading suggests.
• Salinity at 1.025 is fine for a reef. Confirm your refractometer or probe is calibrated, since an uncalibrated reading post-water-change could mean actual salinity shifted more than intended.

Priority Actions

1. Test your replacement saltwater source for ammonia, nitrate, and pH right now. If you mixed a new batch, confirm it was fully aerated and temperature-matched before it went in.
2. Perform another water change of 20-25% with properly matched water (temp 78-79 F, salinity 1.025, pH 8.0-8.3) to pull nitrate down from 80 ppm. Do it slowly. Target below 20 ppm for a mixed reef.
3. Watch the tang for the next 12 hours. If breathing does not ease after the water change, isolate it in a hospital tank. Do not treat in the display tank with any medication while corals and invertebrates are present.
4. Raise alkalinity gradually after nitrate is under control. Dose to bring dKH up by no more than 0.5-1.0 dKH per day to avoid further chemical swings in an already-stressed tank.
5. Identify your nitrate source. At 14 months, 80 ppm in a 55 gallon suggests overfeeding, insufficient export (skimmer output, refugium, or water change frequency), or accumulated detritus. Address the root cause or this will recur.

Watch Closely

• Retest nitrate in 24 hours after the water change to confirm it is dropping.
• Retest alkalinity every 24-48 hours while dosing to avoid overshooting.
• Monitor the tang's breathing rate every 2-3 hours today. If it worsens, is at the surface, or shows white patches or clamped fins, move it to a hospital tank immediately for a freshwater dip and diagnosis.
• Hammer corals should begin opening within 24-48 hours once nitrate drops and alkalinity stabilizes. If they remain closed or show tissue recession by day three, repost with photos for a targeted assessment.