Nutrient balance drives coral color, algae control, and fish health. Most reef issues come from swings, not single numbers. You can stabilize nitrate and phosphate with a simple routine.
Understand your nutrient targets and ratios
Nutrients in reef tanks mainly mean nitrate (NO3) and phosphate (PO4). Corals need both in small amounts. Too low can pale corals and slow growth. Too high can fuel algae and mute colors.
Start with practical target ranges. Aim for NO3 at 2–15 ppm in mixed reefs. Aim for PO4 at 0.03–0.10 ppm. Soft coral tanks can tolerate higher nutrients. SPS tanks often look best near the low end.
Watch the ratio, not just the numbers. If NO3 is 0 and PO4 is 0.10, algae can still thrive. Corals may also starve for nitrogen. If NO3 is 20 and PO4 is 0.00, you can hit dinos and pale tips.
Test on a schedule and log results. Test NO3 and PO4 twice weekly for a month. Then test weekly once stable. Use the same kits each time. Rinse vials with tank water first. For more basics, see reef tank water parameters.
- Mixed reef starting targets: NO3 5–10 ppm, PO4 0.05–0.08 ppm.
- Change goals slowly: adjust no more than 20–30% per week.
- Retest 24 hours after any major change in feeding or filtration.
Control nutrients with feeding, export, and biology
Feeding is the main input. Weigh your feeding habits like a budget. For a 75-gallon reef, start with two small feedings daily. Use a pinch of pellets in the morning. Feed 1–2 frozen cubes at night, rinsed in RO water.
Export is your brake pedal. Skimmers remove dissolved organics before they become NO3 and PO4. Wet skim to lower nutrients faster. Dry skim to maintain stability. Clean the neck every 2–3 days for consistent pull.
Refugiums and algae scrubbers compete with nuisance algae. Run refugium lights 12–16 hours daily. Harvest macroalgae weekly. Remove enough to fill a quart bag. If growth stops, check iron and flow.
Media can fine-tune phosphate. Use GFO or aluminum-based media in small amounts. Start with 25–50% of the suggested dose. Replace when PO4 rises, not by calendar. Carbon helps water clarity and toxin control. Change carbon every 2–4 weeks.
- Skimmer: set for steady foam, not overflow.
- Refugium: keep tumble flow and stable salinity at 1.025–1.026.
- Media: avoid rapid PO4 drops over 0.03 ppm in 24 hours.
Troubleshoot common nutrient problems without chasing numbers
If NO3 and PO4 both read zero, do not celebrate yet. Corals can starve and lose color. Increase feeding by 10–20% for a week. Add more fish only if your tank can support them. Consider dosing nitrate if needed.
If NO3 is high but PO4 is low, stop aggressive phosphate removal. Reduce GFO and increase fish feeding slightly. Check for old rock binding phosphate. A sudden PO4 crash can trigger dinos. Run a UV unit if dinos appear.
If PO4 is high and NO3 is low, you may be carbon dosing too hard. Bacteria can strip nitrate first. Reduce vodka or vinegar dosing by 25%. Increase aeration and skimming. Feed a bit more protein to raise NO3.
Use water changes as a stabilizer, not a reset button. Do 10% weekly for four weeks when nutrients drift. Match temperature within 1°F. Match salinity within 0.001. For a step-by-step routine, read weekly reef maintenance checklist. For algae tactics, see reef algae control guide.
- Cloudy water after changes can mean bacterial bloom. Increase oxygen and stop carbon dosing.
- Green hair algae often means PO4 above 0.10 ppm. Increase export and reduce overfeeding.
- Cyano often follows low flow and dirty sand. Increase flow and siphon weekly.
Nutrient balance is a steady routine, not a quick fix. Pick realistic targets and move slowly. With consistent testing and gentle adjustments, your reef stays clean and colorful.
Sources: Randy Holmes-Farley, “Phosphate and the Reef Aquarium”; Randy Holmes-Farley, “Nitrate in the Reef Aquarium”; Julian Sprung, “The Reef Aquarium” (nutrient control chapters).
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