Your nitrate at 80 ppm is the immediate problem.

Critical Issues

Your nitrate at 80 ppm is the immediate problem, and the timing confirms it: the water change introduced or revealed a condition that is now stressing both your hammer corals and your tang. Hard breathing in a tang is a physiological stress response. At 80 ppm nitrate, a fish already working harder to osmoregulate is a fish that can decline quickly. Do not wait to see if it improves on its own.

The closed hammers reinforce the same diagnosis. Euphyllia ancora and related hammer species are reliable early-warning animals, and corals that were open before a water change and closed after it point directly at water quality as the trigger.

One additional possibility worth taking seriously: if your replacement water was not fully mixed or temperature-matched before the change, a salinity or temperature spike could have compounded the nitrate stress. Your current salinity of 1.025 and temp of 78.5 F are both acceptable, but confirm your refractometer or hydrometer is calibrated.

Secondary Concerns

• Alkalinity at 7.1 dKH is on the lower edge of the acceptable range for a mixed reef (most consensus guidance targets 8.0-9.5 dKH for mixed systems). It is not the cause of this weekend's crash, but it leaves your SPS and LPS with less buffer for growth and tissue repair.
• pH at 8.1 is fine and needs no action.
• With nitrate this high, phosphate is almost certainly elevated as well, even if you have not tested it yet. Elevated phosphate accelerates algae growth and suppresses coral calcification.

Priority Actions

1. Test your source water immediately. Pull a sample of the water you used for the change and test its nitrate. If it reads high, your problem is the source (RO membrane failure, exhausted DI resin, or tap water contamination). This determines your next move.
2. Do a second water change of 15-20% today using confirmed clean water (nitrate 0 ppm, salinity matched to 1.025, temperature matched to 78.5 F). Do not do a large single change, as a rapid nitrate swing can add stress on top of stress.
3. Watch the tang continuously for the next few hours. If breathing worsens, gilling becomes visible and rapid, or the fish stops swimming actively, you need to be prepared to move it to a hospital tank. A tang in respiratory distress from water quality can crash within 24 hours.
4. Repeat 15-20% water changes every 2-3 days until nitrate is below 20 ppm, which is the practical upper target for a mixed reef. Below 10 ppm is better for hammer corals long-term.
5. Bring alkalinity up gradually once the nitrate crisis is resolved. A raise of no more than 0.5-1.0 dKH per day avoids the chemistry swings that stress corals further. Target 8.3-9.0 dKH.
6. Test phosphate as soon as possible. If it is elevated (above 0.05-0.1 ppm for a reef), address it in parallel with nitrate reduction using a refugium, quality carbon dosing, or GFO, depending on your existing equipment.

Watch Closely

• Tang respiration rate: Recheck every 2-3 hours today. Normal breathing is calm and regular. Labored, fast, or visible gill movement is a warning to escalate.
• Hammer corals: They should begin opening within 24-48 hours of improved water quality. If they remain closed beyond 72 hours after nitrate drops, re-examine for secondary infection or a separate chemical irritant introduced during the water change (new equipment, container residue, or additives).
• Retest nitrate, ammonia, and phosphate after each water change to track the curve and confirm your source water is clean.
• Alkalinity: Retest every 2-3 days while you are actively adjusting it, and do not supplement until the tank has stabilized from this event.