đ pH Crash
A pH crash occurs when aquarium pH drops rapidly, usually due to depleted buffering capacity (KH). This is dangerous because fish cannot adapt quickly to pH changes, and acidic conditions can cause ammonia toxicity dynamics to shift. Understanding what causes pH crashes and how to prevent them is essential for stable aquariums.
đ Quick Facts
| Definition | Rapid drop in pH, usually below 6.0 |
|---|---|
| Cause | Depleted KH (carbonate hardness/alkalinity) |
| Severity | High - Can stress or kill fish |
| Prevention | Maintain adequate KH, regular water changes |
| Applies To | Both freshwater and saltwater |
đ Signs of pH Crash
- pH test reads very low - Below 6.0 (freshwater) or 7.8 (saltwater)
- Fish gasping - Stress response
- Lethargy - Fish less active
- Loss of appetite
- Faded colors - Stress indicator
- Erratic behavior
- Sudden fish deaths - If crash is severe
â ī¸ What Causes pH Crashes
Low KH (Buffer Depletion)
The primary cause of pH crashes:
- KH (carbonate hardness) buffers pH
- When KH is consumed, pH can drop rapidly
- Acid-producing processes use up KH
- Without buffer, pH becomes unstable
Acid-Producing Processes
- Nitrification - Beneficial bacteria produce acid
- CO2 injection - Dissolves as carbonic acid
- Decomposition - Organic matter breakdown
- Driftwood/leaves - Release tannins (acidic)
- Certain substrates - Some lower pH
Other Factors
- Soft source water - Low KH to begin with
- Infrequent water changes - Don't replenish buffers
- Overstocking - More acid production
- Heavy feeding - More waste = more acid
đ Emergency Response
Important: Raise pH Slowly!
Do NOT rapidly raise pH - this is as dangerous as the crash itself. Fish adapted to low pH will be shocked by rapid increase.
Step 1: Partial Water Change
- 10-15% water change with properly buffered water
- This slowly raises pH and adds KH
- Repeat every few hours if needed
- Target raising pH no more than 0.2-0.3 per hour
Step 2: Add Buffer (Carefully)
- Seachem Alkaline Buffer (freshwater)
- Baking soda (sodium bicarbonate) - 1 tsp per 10 gal raises KH ~2
- For saltwater: proper reef buffer or kalkwasser
- Add slowly, test frequently
Step 3: Increase Aeration
- Drives off excess CO2
- CO2 is acidic when dissolved
- Helps pH recover naturally
Step 4: Remove Acid Sources
- If using CO2, reduce or stop temporarily
- Remove decomposing matter
- Remove driftwood if contributing
đ pH and KH Guidelines
Freshwater
| Parameter | Target Range |
|---|---|
| pH | 6.5-7.5 (species dependent) |
| KH | 4-8 dKH minimum |
Saltwater/Reef
| Parameter | Target Range |
|---|---|
| pH | 8.1-8.4 |
| Alkalinity | 7-11 dKH |
đĄī¸ Prevention
- Test KH regularly - Weekly at minimum
- Maintain adequate KH - Don't let it drop too low
- Regular water changes - Replenish buffers
- Know your source water - Test tap/RO water
- Use appropriate substrates - Crusite/aragonite buffer in saltwater
- Don't overstock - Less bio-load = less acid production
- Monitor CO2 systems - pH controller recommended
â Common Mistakes
- Raising pH too quickly (shocks fish)
- Only testing pH, not KH
- Using unbuffered RO water
- Ignoring KH depletion until crash occurs
- Adding too much buffer at once
- Not doing regular water changes
đ Summary
pH crashes occur when buffering capacity (KH) is depleted, allowing acid-producing processes to drop pH rapidly. Prevention is key: test KH regularly and maintain adequate levels through water changes and buffering products. If a crash occurs, raise pH slowly through small water changes - rapid correction is as dangerous as the crash itself. In reef tanks, maintaining alkalinity is especially critical due to calcium/alkalinity consumption by corals. Understanding the relationship between KH and pH stability is fundamental to successful fishkeeping.