Complete Guide for Changing Water in Your Fish Tank (With Bonus Tip)

Changing water is one of the first things I learned when I was new with keeping fish. It’s also one of the most important things to do when keeping fish!

A fish tank needs regular water changes for it to be able to keep the balance for your fish and plants. Do it by siphon out 10-30% water and replace it with new water. In the meantime clean algae and trim plants.

This is the single most important part where a lot of people are not doing it as often as they should. It’s the part that takes the longest and could be boring (I think not!) during a week of a fish keeper.

Here’s how to do it correctly and it will go with a breeze, every week!

Why changing the water?

The first step to even do a water change is to understand why you should do it. If you don’t know why you are doing it, you will not do it since you don’t see the point of it.

So, why are we, as a fish keeper, changing the water? We do it because we need to remove ammonia, nitrite, and nitrate from the water. Why we need to remove it is because it’s harmful to the living fish in our fish tanks.

We are not only removing ammonia, nitrite, and nitrate but we are also removing and preventing bad bacteria and parasites that might be in the water making the fish sick. It’s not only making the fish sick, it even kills them.

The ammonia, nitrate, and nitrite are rest-products from the bacteria breaking down the waste from the fish which is needed. It’s called the nitrogen cycle. Cory at Aquarium Co-Op is doing a great job of showing how the nitrogen cycle works, I recommend to watch that video!

If you have bad water conditions for your fish, you run a higher risk of infections, just like you are more likely to get an infection if you are in a dirty environment. By keeping the water clean the bacteria and parasites making your fish sick will not be able to grow, thus having a lower risk of a mishap happening in the fish tank.

All these conditions are not visible for the naked eye and you can not tell just by looking at the fish tank if you need a water change or not. That’s a common thought of a lot of people not doing regular water changes. “I can see when I have to do it!” is a false statement! Don’t believe those persons since it’s those who are going to wake up to a fish tank full of dead fish.

How often should you do water change then? I do it once a week. You could do it every second week but it’s safer to do it every week. You change about 10-30% of the water in the fish tank each change. If you are doing it every week, you could change it closer to 10%. But if you do it every second week it should be about 30%.

If you have any problems in your fish tank, for example, a nitrate spike you should do bigger water changes to remove that nitrate or whatever it is doing the problems in the fish tank. But for the weekly (or bi-weekly) water changes its a good amount to change 10-30% each time.

So, making water changes are vital for the health of your fish tank. Even if the tank seems clean, it might not be and the only way to know the fish tank is clean is to do the regular water changes.


Now you know why you have to do it.

Then start by prepare for the water change. All you need is:

  • A bucket
  • Siphon
  • Water conditioner
  • Somewhere to throw the water away and fill up the new water

You don’t need much. I recommend putting towels on the floor to prevent water damage from spillage when moving buckets filled with water around.

You also need a way to put the bucket higher up then the water level of the fish tank if you don’t want to move a lot of water by hand. More on that later.

What you should never forget before doing a water change is to turn off the air pump and the heater!

If you leave the air pump on while doing water change there will be insufficient water for the pump and the pump will be damaged.

If you leave the heater on during a water change it risks exploding due to quick and large temperature differences. It will turn on and off and the new water will damage it.

Also, be sure to remove any electricity nearby so you won’t damage your electronics if there is water spillage during a water change.

Trimming plants

The next step to do before changing the water is to trim your plants. You will give you filter an easier time to filter the water without the debris from the plant trimming if you do it before you suck out the water and then sucking the debris out from the fish tank while changing the water.

When you trim your plants be sure to trim all plants as the next step. If you found out after the water change to trim a plant, you will get rests from the plant clogging up the filter.

When you have cut a plant, be sure to get the leaves and stems out from the water with the siphon or with your net. Here it’s a great thing to have a multi-box that I recommend on this page. You just put all the leaves and rests on the box and then wash it off. It’s like a third hand!

At this stage, be sure to re-plant your plants and trimmings too! You will always bring up some debris into the water when planting too and it’s best to do it before removing the water.

Cleaning the glass and furniture

When you have trimmed and re-planted your plants, your next step is to remove the algae and dirt that gets stuck on the furniture and on the windows.

To do so, you could use different tools. For the furniture, you could use a cleaning sponge to clean stones, roots and the furniture.

To clean it, just scrub it in the water. What you are cleaning off is dirt and algae that starts to grow. If the algae start to really grow it could be harder to get rid of. And therefore it’s a good idea to do it every week to get rid of the possible small amounts of algae growing and stop it to get a hold of your fish tank.

be sure that the sponge is new and only for the fish tank. Never use the sponge on anything else!

To remove the algae growing on the glass you could use a magnetic algae scraper or an old card of some sort. Be sure, if you use a card it won’t release anything toxic for your fish.

I personally use a magnetic algae scraper but be careful with it. If you go all the way down to the substrate you have a risk of a little stone getting stuck between the algae scraper and the glass. You could imagine what will happen next, it will scratch your glass on the fish tank making it look ugly.

To combat that problem, just don’t go that far down towards the bottom of the glass with the scraper. Instead, use a card to remove the algae near the bottom, if you are about taking that small strip away.

I promise you, no one will see the small strip of dirt towards the bottom. if you have snails they will eat algae on the glass removing that unwelcomed green stuff.

This is wise to do before removing the water to not get the algae and debris into the filter and clogging it up.

Removing the water

To remove the water from the fish tank you use the siphon. With the siphon, you create a vacuum and sucks the water out from the fish tank into the bucket.

To start the siphon, just be sure the tube is into the bucket standing on the floor. Then dip the siphon filling it with water. After the siphon is filled with water, lift it up over the water surface making the water run down into the bucket. As soon as the water starts running put the siphon back into the water quickly making the vacuum and the water flowing into the bucket.

Don’t vacuum the substrate! On the substrate is the good bacteria living breaking down the waste. If you suck them up they will get less and your water condition might get unbalanced. Instead, if your substrate is dirty due to leftover fish food or fish waste, hover the siphon right above the substrate to suck it up.

Otherwise, you should just suck up the floating debris in the water and sucking the water out until you have sucked out the amount of water you want to.

Bonus tip! Water your potted plants with the water you have taken out from the fish tank.

Potted plants are always dying for me. The only plants I am able to keep are the aquatic plants you keep in an aquarium. I have saved a lot of my potted plants by watering them with fish tank water. It’s fertilizer in the water from the fish waste and that’s candy for plants, potted or not.

When you have removed the water and watered your plants you throw it away.

Filling the water

Now you have thrown the water away, it’s time to fill it back up!

You start by filling the bucket with water. The temperature of the water should be the same as in the fish tank. You could feel the water that it’s about the same temperature as in the fish tank. Or even better, you put a thermometer in the water to know exactly what temperature you have.

When filling from a tap, be sure to use cold water and fill the rest with boiling water to get the right temperature. That’s because there’s a risk of heavy metals getting released into the water from the water pipes in higher temperatures. And heavy metals are harmful to your fish.

Speaking of heavy metals, when you have filled your bucket with water you have to put in water conditioner. That’s to remove the metals and the chlorine dissolved in the tap water. Be sure to follow the instructions on the bottle of conditioner.

To get the water from the bucket into the fish tank you could use a mug or something similar to bring the water from the bucket into the tank. This is very time consuming and if you are using this method, be sure to use a mug just for this purpose.

You could also use the same method as you did with removing the water. When doing this method, you have to put the bucket higher then the water level in the fish tank for this to work.

When it’s placed higher you put in a tube (it could be the same tube as you use with the siphon) into the bucket and the other end in the fish tank and start to suck the end going into the fish tank. You suck with your mouth until the water starts to come out. now you have a flow that will continue as long as you have water in the bucket.

If you got any water in your mouth while sucking, be sure not to spit it out in the fish tank. Spit it out in the sink and rinse your mouth with water afterward.

If you have to do multiple trips you just repeat these steps for each bucket.

Now the water is changed and the water level is at normal again. Now you just have to take all the tools away and don’t forget to put the air pump and heater on again!


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

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