Water Changes: The Science of Dilution

If there is one universal truth in fishkeeping, it is this: water changes solve almost everything.

Why Water Changes Work

Your aquarium is a closed system. Nothing leaves unless you remove it. Over time, waste products accumulate:

• Nitrates: Build up from the nitrogen cycle.
• Phosphates: Build up from food.
• Dissolved Organics: Proteins, hormones, pheromones.
• Heavy Metals: Trace amounts from tap water and equipment.
• Growth-Inhibiting Hormones: Fish release hormones that stunt the growth of other fish in the same water.

A water change physically exports these substances and replaces them with fresh, clean water that also replenishes KH buffer and trace minerals.

How Much and How Often?

The Standard Rule

25-30% per week for most community tanks.

High Bio-Load

• Goldfish, Oscars, large Cichlids: 50% per week.

Planted Tanks

• High-tech (CO₂ injected): 50% per week during the first 4-8 weeks, then 30%.
• Low-tech: 20% every 2 weeks may suffice.

Shrimp Tanks

• 10-15% per week. Shrimp are sensitive to parameter swings. Smaller, more frequent changes are better than large infrequent ones.

The Science of Dilution

Each water change removes a percentage of pollutants, not a fixed amount.

Example (starting Nitrate: 40 ppm):

• After 25% change: 30 ppm remaining
• After 50% change: 20 ppm remaining
• After 90% change: 4 ppm remaining

To get from 40 ppm to near zero, you would need multiple back-to-back changes — a single change cannot remove everything.

Water Aging

Some advanced hobbyists "age" their water before use:

1. Fill buckets with treated tap water (dechlorinated).
2. Add an air stone for 24 hours to gas off dissolved gases and equalize temperature.
3. Check parameters (pH, TDS, Temperature).
4. Use — the water is now stable and safe.

This eliminates the risk of temperature shock, dissolved gas supersaturation, and pH swings during the change.

Common Mistakes

• Changing too much at once: Swapping 90% in a tank that hasn't been maintained in months causes osmotic shock.
• Cleaning the filter at the same time: Never deep-clean your filter on the same day as a large water change. You'll crash the bacterial colony.
• Not matching temperature: Cold water dumps can trigger Ich outbreaks.