How to Change Water in a Fish Tank?


Changing your fish tank water is an essential part of keeping your fish healthy and alive. Water changes remove un-seen toxin and chemical build up. Regular water testing will show you how often to change your fish tank water. For more helpful information on how and what kind of water test kits to use to maintain healthy water conditions please see our article, How to Test the Water in an Aquarium.

Fish tank water changes require cleaning the waste material and debris from the bottom of the tank while simultaneously removing and disposing of 10% – 25% of the water from your tank. Use a gravel vacuum to remove waste material and debris from the bottom of your tank without draining more than 25% of the water. Fill your tank back up with water treated with stress coat water conditioner and after a few days, clean your filter.

Why Does my Fish Tank Need Regular Water Changes?

Regular water changes keep your fish healthy and are one of the simplest ways to reduce the overall need for critical maintenance events. Without timely water changes the act of feeding our fish and the creation of fish waste increases nitrate levels to unsafe levels for the occupants of your fish tank.

When nitrate levels are close to 40 parts per million or above it is time for a water change. These waste products can create bacterial imbalances that lead to rising ammonia levels in the water, algae blooms and stinky water (which I cover in the article Why my Fish Tank Smells) all of which is stressful for the occupants and can lead to sickness and death.

Changing the Water and Cleaning Your Fish Tank are Different Things

A water change is not the same thing as cleaning your fish tank. Cleaning your fish tank more completely scrubs away algae, removes waste and debris from all surfaces and also includes a water change. For more helpful information on the general aquarium cleaning process please see our article, How to Clean a Fish Tank. The goal of keeping a healthy and happy fish tank is due to our ability to imitate nature. Nature performs water changes through rainfall that washes waste downstream

How Often do I Need to Perform a Water Change?

Every fish tank is different and only the results of regular water testing will let you know how often you need to perform a water change. Getting into the habit of testing your water once a week will clue you into how much the nitrates are rising.

Remember we want to keep nitrates below 40ppm. So, if your fish tank produces 10ppm of nitrates per week then after 4 weeks our water test will show a nitrate level of 40ppm and you know it is time to perform a water change. If you perform a 25% water change. This will reduce our nitrates from 40ppm to 30ppm.

Based on your weekly water test trend of 10ppm nitrate production you will need to perform another water change in a week. Most fish tanks require a 10% to 25% water change every 1 or 2 weeks depending upon number of live plants and ratio of number of fish to gallons of water.

Bio-Load Drives the Water Change Schedule

It is possible to reduce how often you need to change your water but you need to understand the principal of bio-load. Your fish tank bio-load is what determines how many nitrates are generated and consequently how often you require a water change. The bio-load is determined by the amount of waste your fish are creating in the size fish tank you have.

An overpopulated fish tank will rapidly increase the waste production. Plastic plants do nothing to reduce the bio-load but the benefits of live plants may contribute to significantly reducing the nitrate balance.

Increased Bio-Load From Over Feeding

Bio-load may also be attributed to over feeding your fish. The un-eaten food settles on the bottom of the tank and in hard-to-reach crevasses and begin to rot and ferment which add to the bio-load. Feeding your fish only what they will eat will reduce the ammonia and nitrate levels in your fish tank and the slower your water test levels climb the less often water changes are necessary.

A good way to determine how much food to give your fish is to see if there is any food left after 15 to 20 minutes. If there is still visible food then reduce the amount you are feeding them until you observe all food is eaten and nothing is left floating on the water after they have stopped feeding.

Increased Bio-Load From Fish to Tank Size Ratio

You can also reduce how often you need to change your water by adjusting the fish to tank size ratio. For example, increasing the size of your tank while maintaining your current fish population or just reducing your fish population that share the waste burden over more water which will results in reducing nitrate levels to fewer parts per million.

Expert Tip: If you add live plants to your fish tank you will extend the time needed between water changes as the ecosystem you are creating reduces nitrates naturally.

Increased Bio-Load From Water Evaporation

To reduce the fish tank bio-load you need to maintain the water level. Evaporation is a normal part of the cycle in a fish tank just as it is in nature. As the water evaporates, the amount of water for your fish decreases, which can make the water quality also decrease as more fish waste becomes more concentrated in less water. In nature rain fills the loss from evaporation and you mimic this process by supplementing your fish tank with clean dechlorinated water which will help keep your fish healthy.

In nature there is a consistent source of vitamins, minerals and nutrients. Water changes slowly remove and deplete these essential elements so in addition to the water change and dichlorination treatment it is important to supplement these essential nutrients back into your fish tank.

The supplement needs are different for freshwater and saltwater tank chemistry, so I recommend taking a look at the recommended products page for more specific product guidance.

These efforts may reduce the frequency of needed water changes but will never completely eliminate the need. It is only a matter of time before you will need the next water changes to keep your fish tank healthy.

When Performing a Water Change Do I Remove Everything from the Fish Tank?

Regular water changes do not require the removal of plants, rocks or decorations from the fish tank. Removing large items from your fish tank will stress your fish and may have a negative effect on the balance of beneficial bacteria available in your tank.

The beneficial bacteria reduce the bio-load in your tank. The beneficial bacteria call their home the many surfaces in the tank like rocks, plants, substrate, etc. and help your filter as a natural part of the biological filtration system.

How to Change the Water in a 10 Gallon Fish Tank

If your tank is 10 gallons or smaller using a gravity fed syphon to clean the gravel may remove too much water. But there is an effective alternative that is economical, easy and takes less than 15 minutes to complete.

First use a filtered electric gravel cleaner to remove waste and debris from the bottom of the tank. Some of these systems are multi-functional and can adjust the flow from either re-circulating the water from gravel cleaning back into the tank or to redirecting the flow of water out of the tank in into a buck for disposal. This is a really handy feature that simplifies water changes.

Some of the filtered electric gravel cleaner systems will ONLY recirculate the gravel water through a filter. The best options on the market offer adjustable suction for controlled water changes and the filtration function built with a brush head for cleaning glass and decorations as well as a vacuum tube for cleaning the substrate.

Cleaning fish tank gravel is an important part of maintaining a healthy aquarium environment. The frequency at which you should clean your fish tank gravel depends on several factors that we cover in our article, When to Clean Fish Tank Gravel.

One of our favorite electric vacuum tools is made by Hygger. It is the 360GPH Electric Aquarium Gravel Cleaner. It is a 5 in 1 automatic fish tank cleaning tool set with adjustable vacuum flow and adjustable length. You can even reverse the flow and fill the tank from a clean water source.

Once the gravel cleaning is complete you will need ensure 10% to 25% of the water has been removed and then replaced with clean dechlorinated water.

How to do Water Changes in a Large Aquarium

It is possible to use the same small fish tank water change method for your large freshwater fish tank, but you will find it is very time consuming and messy. For larger tanks there is a much better way that significantly reduces the time it takes to perform a quality water change and without the mess.

The best way to perform a water change in a large fish tank drains the water from your tank directly to your kitchen sink. On one end you use the gravel vacuum to remove debris from the bottom of your tank and the other end of your tube is connected to your sink faucet which creates a syphon. The running water from your faucet syphons water from your fish tank into the sink drain. The tubing typically comes in 25 ft and 50 ft lengths.

There are several products on the market but the Python water changing system is clearly best solution on the market and our number one recommended choice. There are several reason for our recommendation, the most significant of which is the Python water changing system wastes significantly less faucet water than the two most popular water changing systems we compared it to on the market made by Aqueon and Hyger.

Our observations are as follows:

The Python has thicker tubing and couples with sturdy plastic and brass fittings. In comparison the Aqueon and Hyger water changing systems have less sturdy tubing and all fittings seem to be a lower quality/thickness plastic.  The most significant difference in water efficiency was between the Python and Aqueon water changing systems. The Python used 2 gallons of faucet water to drain 1 gallon of fish tank water whereas the Aqueon used 3 gallons of faucet water to drain 1 gallon of fish tank water. They Hyger used about 2 and a half gallons of faucet water to drain 1 gallon of fish tank water.

Python Water Changing System

Re-filling the fish tank is as simple as turning the flow valve attached to your faucet from syphon to fill. This will change the flow of the water to fill the tank directly from your sink.

This convenient solution does not have a way to add dechlorinating chemicals to the water before refilling the fish tank. To solve this problem, we recommend adding an under-sink Reverse Osmosis (RO) system to the faucet you will be using to perform the water change. This will remove the sulfur, lead, iron, manganese, chlorine, fluoride and other toxic chemicals from your tap water before it is added to your fish tank. After your fish tank is re-filled with clean water, treat with stress coat water conditioner and after a few days, clean your filter.

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