Reef Tank Nutrients: Finding the Sweet Spot (and Keeping It There)

If there’s one topic guaranteed to spark debate in the reef aquarium hobby, it’s nutrients. For years, reefers chased “ultra-low nutrients” like a badge of honor. Then corals started paling, tanks stalled, and suddenly everyone was dosing nitrate.

So what’s the truth?

The reality is simple but nuanced:

👉 Corals need nutrients — just not too much, and not too little.

This post breaks down what reef nutrients actually are, ideal target ranges, how to test them accurately, and how to maintain stability without chasing numbers.

In reefkeeping, when we talk about nutrients, we’re usually referring to inorganic nitrogen and phosphorus, primarily:

• Nitrate (NO₃⁻)

• Phosphate (PO₄³⁻)

These nutrients fuel:

• Coral metabolism (via zooxanthellae)

• Bacterial populations

• Microfauna and algae

• The entire reef food web

They enter the system through:

• Fish food

• Fish waste

• Dying algae and detritus

• Additives and supplements

• Some salt mixes and source water

Nutrients are not inherently bad — imbalance is the problem.

There is no single “perfect” number, but decades of collective experience point to safe, productive ranges:

Nitrate (NO₃)

• Soft coral & mixed reefs: 5–15 ppm

• LPS-dominant reefs: 5–20 ppm

• SPS-dominant reefs: 2–10 ppm

Phosphate (PO₄)

• Most reef tanks: 0.03–0.10 ppm

• SPS-heavy systems: 0.02–0.08 ppm

⚠️ Important: Stability matters more than precision.

A stable tank at 10 ppm nitrate will outperform a tank bouncing between 1 and 5 ppm every week.

Nitrate Testing Options

Manual Test Kits

• Reliable brands with fine resolution are preferred

• Ideal for weekly or biweekly testing

• Watch for user error (timing, shaking, lighting)

Digital Testers

• Faster and more repeatable

• Still require calibration and clean cuvettes

• Great for trend tracking

Automated Testers

• High-frequency data

• Best used for trend analysis, not blind trust

• Always verify periodically with a manual kit

Phosphate Testing Options

Phosphate testing is more sensitive and easier to mess up.

Key tips:

• Use ultra-clean glassware

• Avoid touching cuvettes

• Rinse with tank water before testing

• Don’t chase single-test results — watch trends

Low-range phosphate tests are essential. If your kit can’t read below 0.1 ppm, it’s not suitable for reefkeeping.

Nitrate and phosphate are linked biologically. Problems arise when they fall out of balance.

Common scenarios:

• High nitrate + near-zero phosphate → coral starvation, dinos

• High phosphate + low nitrate → algae dominance, poor coral coloration

• Both bottomed out → pale corals, stalled growth, dinos

• Both elevated but stable → often fine (within reason)

A healthy reef exports nutrients proportionally.

If Nutrients Are Too High

Reduce Input

• Feed more efficiently

• Avoid overstocking fish

• Rinse frozen foods if needed

Increase Export

• Protein skimming

• Refugiums with macroalgae

• Mechanical filtration (filter socks, rollers)

• Regular water changes

Targeted Media

• Phosphate absorbers (used gently)

• Avoid rapid drops — slow and steady wins

If Nutrients Are Too Low (Very Common!)

Yes — many modern reef tanks are too clean.

Increase Input

• Feed fish more (or more often)

• Add more fish biomass

• Reduce mechanical filtration

Reduce Export

• Run skimmers drier or on a schedule

• Harvest refugium less aggressively

• Remove unnecessary media

Controlled Dosing

• Nitrate or phosphate dosing can be effective

• Dose conservatively and monitor closely

• Match dosing to consumption trends

Low nutrients often look “clean” — until corals slowly starve.

Chasing exact numbers causes more harm than good.

Signs of a healthy nutrient balance:

• Consistent coral coloration

• Visible growth

• Polyp extension

• Predictable consumption patterns

• Minimal nuisance algae

Signs of instability:

• Rapid swings after adjustments

• Constant tweaking

• Corals improving briefly, then declining

• Recurrent algae or dinoflagellate outbreaks

The best reefkeepers don’t test to react — they test to confirm consistency.

Nutrients don’t exist in isolation. Their impact depends on:

• Lighting intensity and spectrum

• Flow

• Alkalinity stability

• Trace element availability

• Biological maturity of the tank

A low-nutrient system under weak light behaves very differently than one under intense lighting.

Context matters.

Modern reefkeeping has taught us an important lesson:

Starving a reef is just as dangerous as polluting it.

Healthy reef tanks embrace nutrients, manage them intelligently, and prioritize long-term stability over short-term control.

Test consistently. Make small changes. Watch your corals more than your test kits.

When nutrients are right, reefs don’t just survive — they thrive.