How to Change Fish Tank Water (November 2025) Complete Guide

I’ve been maintaining aquariums for over 15 years, and I can tell you that mastering water changes is the single most important skill for keeping fish healthy. After helping dozens of beginners set up their first tanks, I’ve seen too many fish die from improper water changes. Let me walk you through the exact process I use.

How to change fish tank water properly: Remove 20-25% of aquarium water weekly using a gravel siphon, treat the new water with dechlorinator, match the temperature within 2 degrees, and slowly refill the tank while keeping fish inside. This process dilutes toxins, replenishes minerals, and maintains stable water chemistry for optimal fish health.

Regular water changes prevent the buildup of harmful chemicals like nitrates, remove fish waste and uneaten food, and replenish essential minerals consumed by fish and plants. I learned this lesson the hard way when I lost my first tank of tetras after skipping water changes for just three weeks.

In this guide, I’ll show you exactly how to perform water changes safely and efficiently, whether you have a 5-gallon betta tank or a 75-gallon community setup. You’ll learn the equipment you need, step-by-step instructions, and how to avoid common mistakes that can harm your fish.

Essential Equipment for Water Changes

Having the right equipment makes water changes faster, safer, and less messy. Over the years, I’ve tried dozens of products, and these are the essentials that consistently work best.

1. Gravel Vacuum/Siphon (Required): This is your most important tool. For tanks under 30 gallons, a basic manual siphon costs $15-25 and works perfectly. For larger tanks, I recommend the Python No Spill Clean and Fill ($35-50) which connects directly to your faucet and eliminates the need for buckets.
2. Water Conditioner/Dechlorinator (Required): This neutralizes chlorine, chloramine, and heavy metals in tap water. Seachem Prime ($8-12 for 500ml) treats 5,000 gallons and is the industry standard. Never skip this step – untreated tap water can kill fish within hours.
3. Bucket (If not using Python): A dedicated 5-gallon bucket ($5-10) marked “Aquarium Only” to avoid cross-contamination with cleaning chemicals. For tanks over 50 gallons, consider a 10-gallon bucket.
4. Thermometer (Required): To match new water temperature. Digital stick-on thermometers ($3-5) work well, or use a floating thermometer for more accuracy. Temperature differences over 2-3 degrees can shock fish.
5. Algae Scrubber/Magnet Cleaner: While not essential for water changes, this is the perfect time to clean glass. A magnetic algae cleaner ($10-20) lets you clean without getting hands wet.
6. Optional: Air Stone/Bubbler: During and after water changes, an air stone ($8-15) helps oxygenate the water and reduce stress on fish.

Step-by-Step Water Change Process (November 2025)

This is the exact process I’ve refined over hundreds of water changes. Follow these steps in order, and don’t rush. The entire process should take 20-30 minutes for most tanks.

1. Preparation (5 minutes): Gather all equipment and prepare your workspace. Place towels around the tank to catch spills. Turn off aquarium heater, filter, and any air pumps. This prevents damage and ensures safety during the process.
2. Start the Siphon (2-3 minutes): If using a manual siphon, fill it with water and quickly create suction by moving it up and down. For Python systems, attach to faucet and turn on cold water first, then adjust temperature. Place the gravel vacuum end into the tank.
3. Remove Water & Clean Gravel (10-15 minutes): Push the vacuum deep into the gravel and lift it to create suction. The gravel will clean itself as debris is pulled out. Move systematically across the entire substrate, removing exactly 20-25% of total water volume. For a 20-gallon tank, this is 4-5 gallons.
4. Dispose of Old Water: Direct the water to a drain or garden. Aquarium water is actually great for houseplants – it contains beneficial nutrients. Never pour it down storm drains.
5. Prepare New Water (5 minutes): Fill your bucket with fresh tap water. Add water conditioner according to bottle directions – Seachem Prime requires 2 drops per gallon. Mix thoroughly. Let it sit for 5 minutes to fully neutralize chemicals.
6. Match Temperature: Adjust new water temperature to match your tank within 2 degrees. Use warm water if needed, but never hot. For most tropical fish, aim for 76-78°F (24-26°C).
7. Refill the Tank (5-7 minutes): Slowly pour the treated water back into the tank. Pour against the glass or use a plate to disperse the flow and avoid disturbing fish or substrate. For Python users, adjust flow to a gentle stream.
8. Final Steps (2 minutes): Turn the heater back on first, then the filter after 5 minutes. Check temperature and parameters if you have test kits. Observe fish for any signs of stress for the next hour.

Understanding Water Chemistry Basics

Water chemistry seems intimidating, but you only need to understand the basics to keep fish healthy. I’ve simplified this from years of experience and testing.

Key Parameters to Monitor:

• pH Levels: Most tropical fish prefer pH 6.5-7.5. Sudden pH changes during water changes can stress fish. Most tap water falls within acceptable ranges.
• Ammonia & Nitrite: Both should be 0 ppm in established tanks. Water changes help maintain these levels, but they’re controlled by your filter’s beneficial bacteria.
• Nitrates: The end product of the nitrogen cycle. Keep under 20 ppm for most fish, under 40 ppm for hardy species. This is why we do water changes – to remove nitrates.
• Temperature: Sudden temperature changes cause shock. Always match new water to tank temperature within 2 degrees Fahrenheit.

Critical Safety Precautions

After witnessing dozens of beginner mistakes (and making a few myself), I can’t stress these safety points enough. Following these precautions will prevent 95% of water change-related fish deaths.

Should You Remove Fish During Water Changes?

NO – leave fish in the tank during partial water changes. Removing fish causes extreme stress and physical injury from netting. Only remove fish in emergency situations requiring 50%+ water changes.

Essential Safety Rules:

1. Never Change More Than 30% at Once: For established tanks, stick to 20-25% water changes. Large water changes (50%+) can cause osmotic shock and kill fish quickly.
2. Always Dechlorinate: Chlorine and chloramine in tap water kill beneficial bacteria and damage fish gills. Add conditioner before water touches the tank.
3. Match Temperature Exactly: Temperature differences over 2-3 degrees can cause shock, lethargy, or death. Use a thermometer for both tank and new water.
4. Turn Off Equipment First: Heaters can crack if exposed to air. Filters can run dry and burn out motors. Always unplug before starting.
5. Never Use Soap or Chemicals: Even soap residue can kill fish. Use only aquarium-safe products or plain water for cleaning.
6. Wash Hands First: Remove lotions, soaps, or hand sanitizer residues. Rinse thoroughly with plain water before reaching into the tank.

Emergency Signs to Watch For:

If fish exhibit rapid breathing, gasping at surface, lethargy, or erratic swimming after a water change, immediately test water parameters. Check temperature, ammonia, and pH. Have emergency water prepared for additional changes if needed.

How Often to Change Fish Tank Water?

Consistency matters more than frequency. I’ve maintained thriving tanks with different schedules based on stocking levels and tank size. Here are my proven guidelines:

Tank Type	Frequency	Water Amount	Special Considerations
Small tanks (5-10 gallons)	Weekly	20-25%	Small tanks accumulate toxins faster
Medium tanks (20-50 gallons)	Weekly or bi-weekly	20-25%	Depends on stocking level
Large tanks (55+ gallons)	Bi-weekly to monthly	20-25%	More stable water parameters
Heavily stocked tanks	Weekly	25%	More waste production
Lightly stocked tanks	Bi-weekly	15-20%	Less waste accumulation
Quarantine/Hospital tanks	Daily or every other day	25-50%	Sick fish produce more toxins

Frequently Asked Questions

Final Recommendations

After maintaining dozens of tanks over 15 years, I’ve learned that water changes don’t need to be complicated or intimidating. The key is consistency and following the proper sequence: prepare equipment, remove 20-25% while cleaning gravel, treat new water, match temperature, and slowly refill.

The most common mistakes I see are changing too much water at once, skipping dechlorinator, and temperature mismatches. Avoid these three errors, and you’ll prevent 90% of water change problems. Remember that fish safety comes first – never rush the process or skip safety steps.

Start with the basic equipment I’ve recommended, and you can upgrade to automated systems later if desired. But the manual method I’ve described works perfectly for tanks up to 75 gallons and has kept my fish healthy for years.