Water Chemistry for Reef Tanks: The Numbers That Matter
A beginner's guide to testing, understanding, and maintaining perfect water conditions
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Why Chemistry Matters
Your reef's health lives in the water. Fish, corals, and invertebrates can't survive in toxic water, no matter how beautiful your tank is.
Good chemistry = Happy animals Bad chemistry = Slow deaths
Let's make sure you get it right.
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The 6 Numbers You Need to Know
1. Salinity (Specific Gravity)
What it is: How "salty" your water is.
Ideal range: 1.023 - 1.026 (or 32-35 ppt)
Why it matters: - Too low (fresh): Animals dehydrate, osmotic stress - Too high (salty): Animals hyperhydrate, can't eat - Just right: Perfect osmotic balance
How to test: Refractometer ($20-40)
How to fix: - Too high → Add freshwater (distilled or RO) - Too low → Add salt mix (small amounts)
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2. pH (Acidity/Alkalinity)
What it is: How acidic or alkaline water is.
Ideal range: 8.0 - 8.3 (alkaline, as it should be)
Why it matters: - Too low (acidic): Corals won't calcify, animals stressed - Too high (too alkaline): Uncommon, can stress animals - Just right: Corals grow, pH stable
How to test: pH test kit or digital meter ($15-100)
Why it drops: - Organic decay (uneaten food, poop) - Respiration (animals consume oxygen, produce CO2) - Not enough water changes
How to fix: - 25% water change (most important) - Add buffer (if severely low) - Improve aeration
Maintenance: Test weekly. Small drops are normal (7-day cycle is expected).
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3. Ammonia (NH₃/NH₄⁺)
What it is: Toxic compound from fish waste and decaying food.
Ideal range: 0 (ZERO)
Why it matters: - Ammonia is deadly - Even small amounts stress fish - Kills fish quickly at high levels - Shows you have a bioload problem
How to test: Liquid test kit ($15-25) — most accurate
Why it appears: - Fish waste (normal in new tanks) - Uneaten food (overfeeding) - Dead animals (remove immediately) - Decaying plants/algae
How to fix: 1. Do NOT feed until it drops to 0 2. Do NOT add fish (let cycle complete) 3. Water change (25%) to dilute 4. Clean substrate (remove debris) 5. Wait 3-7 days for bacteria to process it
If you see ammonia after cycling: - Bioload is too high (too many fish) - Tank is overfed - Filter is clogged - Water changes are skipped
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4. Nitrite (NO₂⁻)
What it is: Intermediate compound in nitrogen cycle. Toxic.
Ideal range: 0 (ZERO)
Why it matters: - Nitrite is toxic (like ammonia) - Shows ammonia is being processed - If present after cycling = problem
How to test: Liquid test kit ($15-25)
The nitrogen cycle: Ammonia → (bacteria) → Nitrite → (bacteria) → Nitrate
During cycling: - Days 0-7: Ammonia high, nitrite low - Days 7-14: Ammonia drops, nitrite rises - Days 14-21: Nitrite drops, nitrate rises - Day 21+: All stable (cycle complete)
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5. Nitrate (NO₃⁻)
What it is: Final product of nitrogen cycle. Less toxic than ammonia/nitrite.
Ideal range: <20 ppm (parts per million)
Why it matters: - Nitrate feeds algae growth - Algae blooms make tank ugly - Very high nitrate (40+) stresses animals - Won't kill immediately, but indicates poor maintenance
How to test: Liquid test kit ($15-25)
Why it rises: - Normal (fish waste becomes nitrate) - Algae isn't consuming it fast enough - Water changes infrequent - Tank overstocked
How to fix: 1. 25% water changes monthly (most important) 2. Reduce feeding (less fish waste) 3. Increase plant/algae (they consume nitrate) 4. Add more filtration (biological media) 5. Clean substrate regularly (remove detritus)
Reality check: If nitrate is 0, your test kit might be broken. Some nitrate (5-15) is normal and healthy.
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6. Alkalinity (dKH / Carbonate Hardness)
What it is: Ability of water to resist pH changes (buffering capacity).
Ideal range: 8-12 dKH (or 2.8-4.3 meq/L)
Why it matters: - Buffers pH swings - Corals need alkalinity to build skeletons - Low alkalinity = pH crashes - High alkalinity = pH stays stable
How to test: Liquid test kit ($20-30) or digital
Why it drops: - Corals consuming it (calcification) - pH buffering (using it up) - Decay processes
How to fix: 1. Water changes (easiest) 2. Add buffer (Reef Advantage Alkalinity powder) 3. Reduce bioload (less decay = less buffering needed) 4. Better aeration (helps pH stability)
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Optional But Useful Parameters
Calcium
Why it matters: Corals build skeletons from calcium + alkalinity
Ideal range: 400-450 ppm
How to fix: Calcium supplement (if low) or water changes
Frequency: Test monthly (for established tanks)
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Phosphate
Why it matters: Feeds algae (bad)
Ideal range: <0.5 ppm
How to fix: Water changes, reduce feeding, algae export
Frequency: Test monthly
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Magnesium
Why it matters: Works with calcium for coral growth
Ideal range: 1200-1400 ppm
How to fix: Usually comes in salt mix, supplements available
Frequency: Test quarterly (rarely out of range)
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Testing Equipment
Budget Setup ($50-75)
The Essentials: - Refractometer ($25) - salinity - Liquid test kit ($30) - ammonia, nitrite, nitrate, pH - Basic thermometer ($5) - temperature
Covers: All critical parameters for beginner
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Mid-Range Setup ($150-250)
Add to budget: - Digital pH meter ($60-80) - more accurate than kit - Alkalinity liquid test ($20) - important for established tanks - Thermometer with probe ($15) - automation-ready
Covers: Everything you need
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Premium Setup ($300-500+)
Add to mid-range: - Auto-testing probe system ($300+) - digital monitoring, alerts - Backup test kits (redundancy) - Phosphate test kit - Calcium test kit
Covers: Everything with redundancy and automation
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Testing Schedule
New Tank (First Month)
| When |
| Daily |
| 2-3x per week |
| Weekly |
| Every 3 days |
Goal: Complete cycling (ammonia & nitrite → 0)
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Established Tank (Monthly Maintenance)
| When |
| Weekly |
| Monthly |
| Monthly |
| Quarterly |
Quick rule: If something looks wrong, test everything.
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Common Chemistry Problems & Fixes
pH Keeps Dropping
Diagnosis: - Test ammonia/nitrite (if present = bioload issue) - Check alkalinity (if low = buffering problem)
Fix: - 25% water change (immediate relief) - Reduce feeding (lower bioload) - Add buffer (Reef Advantage Alkalinity) - Improve aeration (increase water surface agitation)
Prevention: Don't skip water changes.
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Nitrate Won't Drop
Diagnosis: - Fish overstocked (too much waste) - Overfeeding (excess food = excess waste) - Water changes insufficient (not diluting) - Filter clogged (bacteria can't process waste)
Fix: 1. Remove 1-2 fish (reduce bioload) 2. Cut feeding in half (measure portion sizes) 3. Do 50% water change (drastic dilution) 4. Clean filter media (restore bacterial processing) 5. Add more plants/algae (they consume nitrate)
Patience: Nitrate takes weeks to drop. Results show in 3-4 weeks.
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Ammonia Spikes After Cycling
Causes: - New fish added too fast - Large food pieces decaying - Dead animal hiding somewhere - Filter failure
Emergency steps: 1. Stop feeding immediately 2. Do 50% water change (dilute ammonia) 3. Test filter (is it running?) 4. Search for dead animals (under rockwork, etc.) 5. Don't add fish until ammonia = 0 for 3 days
Prevention: Add fish slowly, one at a time, 1-2 weeks apart.
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Salinity Keeps Rising
Cause: Evaporation (water leaves, salt stays)
Fix: - Top off with freshwater only (distilled or RO) - Never add salt water for evaporation - Use auto top-off system (if advanced)
Prevention: Daily check. Top off as needed.
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Water Change Routine
The Why
Water changes dilute: - Nitrate (most important) - Phosphate (algae food) - Unwanted compounds - Replenish trace elements
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The How
Frequency: 25% monthly (or 10% bi-weekly)
Process: 1. Prepare new saltwater (24 hours in advance) 2. Siphon out 25% of tank water 3. Match temperature of new water 4. Add new water slowly (don't blast fish) 5. Wait 15 minutes before testing
Pro tip: Do water changes same day each month (habit)
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Cycling Your Tank (Critical!)
What Is Cycling?
Establishing beneficial bacteria that convert: - Ammonia → Nitrite → Nitrate
No bacteria = deadly ammonia Bacteria established = Safe for animals
Timeline
Week 1: - Set up tank - Add live rock (inoculates bacteria) - Test: Ammonia should rise
Week 2: - Ammonia drops, Nitrite appears - Test every 2 days
Week 3: - Nitrite drops, Nitrate appears - Both ammonia and nitrite = 0
Week 4: - All parameters stable - Ammonia = 0, Nitrite = 0, Nitrate = 5-20 - CYCLE COMPLETE ✅
Don't Rush Cycling
Temptation: Add fish after 1-2 weeks
Reality: You'll kill them
Wait: Full 3-4 weeks, confirm both ammonia AND nitrite are 0
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Product Recommendations
Test Kit Bundle ($75)
Includes: - Liquid test kit (all parameters) - Refractometer - Temperature probe - Calibration solution---
Water Treatment ($49)
Includes: - Alkalinity buffer - Calcium supplement - Trace element solution - Instructions---
Premium Auto-Tester ($399)
Includes: - Digital probe (monitors pH, salinity, temp) - Auto alerts - Logging data - Mobile app---
FAQ: Chemistry Questions
Q: Can I use tap water? A: No. Tap water contains chlorine, phosphate, and unknowns. Use RO water (reverse osmosis) or distilled. Most aquarium stores sell RO water ($1-2 per gallon).
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Q: How do I make saltwater? A: Mix salt (marine salt mix) + RO water. Follow salt bucket instructions. Let settle 24 hours before using.
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Q: My ammonia is 0.25 (tiny bit). Should I worry? A: Yes. Even 0.25 is stressful. Do 25% water change and retest.
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Q: Can I fix chemistry with just supplements? A: No. Water changes are 90% of chemistry management. Supplements are 10%.
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Q: How often do I need to test? A: Weekly during first month, monthly after established.
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The Bottom Line
Master these 6 numbers: 1. Salinity 1.023-1.026 2. pH 8.0-8.3 3. Ammonia 0 4. Nitrite 0 5. Nitrate <20 6. Alkalinity 8-12 dKH
Do this every month: - 25% water change - Full parameter test - Maintenance check
Ignore nothing: - Chemistry problems = animal death - Test when something looks wrong - Better safe than sorry
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Support
Chemistry confused you? We're here to help.
📧 Email: mdyer.nz@gmail.com 📞 Phone: 09 281 1380
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Perfect water chemistry = thriving reef. You've got this. 🧪