Your yellow tang's labored breathing is the most urgent problem right now.

Critical Issues

Your yellow tang's labored breathing is the most urgent problem right now, and it points directly to your nitrate of 80 ppm and phosphate of 0.42 ppm as likely culprits, though a secondary cause deserves equal attention: something about Sunday's water change stressed this tank hard. Two hammer corals closing and a fish showing respiratory distress within the same 48-hour window after a water change is not coincidence. The most common triggers are a significant temperature swing, a pH shock, a salinity mismatch in the new water, or introduction of a contaminant (tap water, bucket residue, a new additive). You need to identify what changed on Sunday.

Nitrate at 80 ppm is acutely harmful to corals and a chronic stressor for fish. Euphyllia species (your hammer corals) are noticeably sensitive to high dissolved organics, and a water change that temporarily stirred up detritus can spike parameters and introduce additional stress on top of an already elevated baseline.

Secondary Concerns

• Alkalinity at 7.1 dKH is below the commonly recommended reef target of 8.0-9.5 dKH. It is not dangerous at this level, but it is low enough to slow coral growth and reduce the buffer available to stabilize pH. Given that your hammer corals are already closed, restoring alkalinity to the 8.3-8.5 dKH range should be part of your recovery plan once the immediate crisis is addressed.
• Calcium at 380 ppm is on the low end of the acceptable reef range (380-450 ppm). It is not an emergency but should be brought up gradually toward 420-430 ppm in parallel with alkalinity correction, since the two are linked.
• Magnesium at 1180 ppm is acceptable ([number removed] ppm is ideal). A modest increase would help calcium and alkalinity hold more stable.
• Phosphate at 0.42 ppm is roughly four times the upper threshold most mixed reef keepers target ([number removed] ppm). This level alone can bleach and close LPS corals over time and is contributing to your nitrate problem by indicating a broader nutrient export failure.

Priority Actions

1. Check your yellow tang closely for gill flukes or ich. Hard, rapid breathing in a tang can be gill flukes (Neobenedenia or Dactylogyrus-type trematodes) or early-stage Cryptocaryon (ich), especially if you see flashing, clamped fins, or white spots. If the fish deteriorates further, prepare a hospital tank with matched water so you can treat without exposing your invertebrates. Do not use copper in the display tank.
2. Review exactly what happened on Sunday. Confirm the temperature, salinity, and pH of the water you added before it went in. Check whether any new product, a different salt mix, or tap water was used. If you used a bucket or hose that previously held soap or chemicals, that is a likely contaminant.
3. Do a 30-35 percent water change today or tomorrow using freshly mixed, temperature and salinity matched saltwater (verified with a refractometer or accurate hydrometer before it goes in). This will dilute nitrate and phosphate meaningfully and may help your tang if the issue is water quality rather than parasites.
4. Address nitrate at the source. Weekly 20 percent changes are not keeping up, which means your bioload or feeding rate exceeds your export capacity. Add or increase a refugium with chaeto, upgrade your skimmer, reduce feeding temporarily, and vacuum detritus from the substrate during every water change.
5. Raise alkalinity slowly. Dose to raise it no more than 0.5-1.0 dKH per day to avoid chemistry shock. Target 8.3-8.5 dKH. Do not raise calcium simultaneously with large alkalinity corrections; stagger them.
6. Add a phosphate export method. A quality GFO reactor (granular ferric oxide) or a phosphate-binding media in a reactor or high-flow area will pull phosphate down more reliably than water changes alone. Bring it down gradually rather than crashing it quickly, as a sudden phosphate drop can also stress corals.

Watch Closely

• Tang breathing: Recheck every few hours for the next 24-48 hours. Any worsening, visible spots, or loss of appetite means move to a hospital tank immediately.
• Hammer corals: They should begin opening within 24-48 hours of stable, improved water conditions. If they remain fully closed or show recession of tissue from the skeleton after 72 hours, tissue damage is occurring.
• Retest nitrate and phosphate 48 hours after your next water change to confirm the trend. If nitrate is still above 60 ppm after a 30 percent change, you have a significant detritus or overfeeding problem that needs a more systematic fix.
• Retest alkalinity 24 hours after any dosing adjustment to confirm the rise is controlled and consistent.