Nitrate at 80 ppm is the immediate problem.

Critical Issues

Nitrate at 80 ppm is the immediate problem. That level is acutely stressful for a mixed reef and almost certainly explains both the closed hammer corals and the tang's labored breathing. Hammer corals (Euphyllia ancora) begin showing stress well below this level, with extended polyp retraction typically appearing above 20-40 ppm. A tang showing respiratory distress at 80 ppm nitrate is in a genuinely dangerous situation, not just a suboptimal one.

The timing points directly at the water change. Two possibilities need to be ruled out fast:

• The new water introduced a contaminant (chloramine breakthrough, metals, or phosphate spike that crashed oxygen levels)
• The water change was done with a different salinity or temperature and caused osmotic stress, compounding an already high-nitrate environment

Check your alkalinity as well. At 7.1 dKH you are on the low end for a mixed reef, and a swing in alkalinity around the water change could be contributing to the coral closure independently of the nitrate.

Secondary Concerns

• 7.1 dKH alkalinity is not dangerous on its own, but the target range for a mixed reef is generally 8.0-9.5 dKH. If this dropped noticeably from your previous reading, the swing itself is a stressor for Euphyllia even if the absolute number looks passable.
• pH at 8.1 is fine and not a concern at this time.
• Salinity at 1.025 is appropriate. No issue there.

Priority Actions

1. Test the tang for gill flukes or other parasites right now. Respiratory distress in a tang is not always water quality alone. Look for rapid operculum movement, flashing, or mucus on the gills. If flukes are suspected, a freshwater dip (matched pH and temperature, 5-7 minutes) can serve as both a diagnostic and a short-term treatment. Do this in a separate container, never the display tank.

2. Do an immediate partial water change of 20-25% using water you have verified for temperature match (78-79 F), salinity match (1.025), and dechlorination. This will bring nitrate down toward 60 ppm in one step. Do not do a massive single change, as a sudden nitrate crash can also stress livestock.

3. Test your tap or RO source water directly for ammonia, chloramine, and pH. If you use RO/DI, check your TDS meter. A failed membrane or exhausted DI resin can introduce contaminants that would not show up on a standard nitrate test but would affect fish respiration and coral behavior.

4. Raise alkalinity gradually toward 8.3-8.5 dKH. Do not dose more than 0.5-1.0 dKH per day to avoid a swing that stresses corals further.

5. Identify and address the nitrate source long-term. At 14 months and 80 ppm, you likely have accumulated detritus, an overloaded skimmer, or an undersized refugium/media reactor. This level will suppress coral health chronically even after the immediate crisis passes.

Watch Closely

• Tang breathing: Recheck in 2-4 hours after the water change. If respiratory rate does not improve, or if you see other fish affected, the cause is more likely a water contaminant than nitrate alone, and further source-water investigation is urgent.
• Hammer corals: Expect 24-48 hours before you see extension returning after nitrate drops. If they remain closed beyond 72 hours post-correction, test for a separate alkalinity swing or physical damage.
• Nitrate: Retest 24 hours after each water change to track the reduction curve. Target below 20 ppm before considering the situation resolved.
• Alkalinity: Test every 48 hours while dosing upward, and compare against your pre-water-change reading if you have one recorded.