Reef Tank Water Change – Your First Time Guide!

You may have been doing water changes on your freshwater aquariums for years, or this is your first ever aquarium, but now its getting time to think about your first saltwater change and you have so many questions!

Your first reef tank water change requires high-quality source water that is heated to 78°F & mixed with synthetic salt crystals designed for saltwater aquariums. After the water has mixed for at least 1 hour, the temperature and salinity match that of the aquarium, it is ready for changing.

Some of the question you may have before your first saltwater water change are:

  • How Much Salt Do I Use?
  • What Kind of Salt Do I Use ?
  • What Kind of Water Do I Use ?
  • Do I Vaccum The Sandbed?
  • Do I Do Water Changes During The Cycle?
  • and many more…

This article will guide you through everything you need to know and how to do your first water change on your saltwater aquarium.

Let’s get those questions answered!

How To Change Aquarium Water

The one thing you must do is:


This will burn any fish, invertebrates or corals that the granule lands on, fish can eat the granules then die and you will create very high areas of salinity in the local area of the salt pile.

You are going to need the following items:

  • Vessel of water with the correct volume in it. Eg if you are removing 10% of a 55 gal aquarium, you need 5.5 – 6 gal of water in the vessel
  • Another Identical vessel – Only for the first time doing a water change!
  • Bag/Bucket of selected salt mix
  • Sandbed/gravel Vacuum – Like this Python at
  • Turkey Baster
  • Measuring cups/spoons for use with baking
  • Thermometer
  • Refractometer
  • Small Powerhead
  • 6-10ft length of hose to fit on the powerhead output nozzle
  • Small Heater
  • Glass brush/scraper – Like this Magnetic Scraper at
  • Toothbrush
  • Old Towels – 2 Minimum
  • Permanent Marker Pen

STEP 1 – Protect The Floor

Place down an old towel on the floor in front of your aquarium to protect the floor. You will spill and splash one day! Keep the other towel close by for drying your hands and wiping up drips.

STEP 2 – Place Your Water Change Vessels

Place your two vessels next to one another at the side of the tank or close by. You need the hose to reach, but you need access to each glass panel on your aquarium.

STEP 3 – Prepare The Saltwater Container

Fill one of the vessels with the required volume of filtered water and in it place the small powerhead and the heater. No hose on the powerhead required yet. Set your heater to the same temperature as your aquarium water. Turn on the pump and heater.

STEP 4 – Add The Salt

Read the instructions on your salt container and measure out the required amount of salt using your measuring cups/spoons.
Instant Ocean is 1 cup for every 2 US Gallons, so a 6 gal water change requires 3 cups of salt granules. Sprinkle the salt into the vessel with the water in it and leave to thoroughly mix and heat. Allow mixing for at least 1 hour if possible. A few hours is recommended.

STEP 5 – Clean The Aquarium

While your salt is mixing this is when I get my aquarium all dirty! Clean all your glass with the scraper and brush. I use the toothbrush to clean all the seams and corners. Scrub all your wavemakers and overflow/s too to keep them sparkling.
Take your turkey baster and blast all your rocks to disturb any trapped detritus and get it into the water column.

STEP 6 – Check The Saltwater

Now is time to check your salt mix. Is it up to temperature? Has all the salt dissolved?. Check the temperature with your thermometer and your salinity level with a Refractometer.

==> Read My Article Here On Measuring Your Salinity Level <==

If your salt is not ready, go and have a beer/wine/coffee and let it mix some more.

STEP 7 – Make Any Adjustments

Make any adjustments to the new saltwater mix.
If your salinity reading is too low add a little bit more salt granules.
If your salinity reading is too high add a little bit more freshwater.

Leave to mix and repeat until your new mix water exactly matches your aquarium water.

STEP 8 – Prepare To Change The Water

Once your water mix is ready its time to change the water. Turn off any powerhead/wavemaker in your aquarium that may be left out of the water when you drain some of the tanks water.

STEP 9 – Vacuum The Sandbed

Take your sand vacuum and place the end of the hose into the other, empty vessel you have sat next to the new saltwater mix. Start the siphon and vacuum the sand. Water will start to fill the empty vessel. Keep draining the aquarium water until the water in both vessels is the same height.

Here is a quick video on how to use a Gravel/Sand Vacuum:

STEP 10 – Mark The Water Level

You have now removed the same amount of water as what you will place back in. Take your Permanent Marker and in a hidden place (usually at the back corner), mark a line on the outside glass where the water level is in the aquarium. This is now where you will drain your water to for each water change. You will no longer need to drain into a vessel, you can drain straight into the sink after today!

STEP 11 – Fill With New Saltwater

Turn off the powerhead and heater in the mixing vessel and remove the heater.
Place the 6-10ft hose onto the outlet of the powerhead and place the other end into the aquarium.
Turn on the powerhead and fill the aquarium with the new saltwater mix.
Be sure not to direct it into the sandbed and create a dust cloud!

STEP 12 – Get Every Last Drop

Fill the aquarium until the powerhead is unable to pump anymore. Use a cup or jug to pour the remaining new saltwater mix into the aquarium. You have now just completed your first Saltwater Aquarium Water Change!

STEP 13 – Turn Back On Equipment

Be sure to turn on all wavemakers and equipment on the aquarium that you turned off before removing the water.

STEP 14 – Replace Canopy

Clean and replace all your canopy and clean the outside glass with a soft cloth.
Pack away all your equipment and dispose of the old aquarium water.

STEP 15 – Sit Back & Relax!

Sit back and relax while watching your aquarium. Your water may be a little cloudy for a couple of hours but then it will be crystal clear until next time!
Be sure to clean or change your filter floss/socks the next day after they have collected all the suspended detritus that was in your water.

Why Do You Need To Do An Aquarium Water Changes?

You may have seen people on the forums and social media telling everyone they don’t do water changes and have not done one for years and they have had no trouble! So why am I telling you to do one? One word, HEALTH.

How many times have you been stuck indoors all day in a room full of people and the air is just stuffy, full of germs and you are just dying to get outside and into the fresh air? Is it harming you to be in that room? Probably not, but would you like to live like that all day, every day? Heck No! So why would your fish?

Your aquarium is a closed ecosystem that tries to reach an equilibrium between maintaining health and being poisoned.
As your livestock do their day to day activities, they go to the bathroom, exhaust Carbon Dioxide, and Nitrogen is produced by the beneficial bacteria in your biological filter.

If your bacteria are unable to cope with converting these toxins they will eventually build up to levels which will poison every living thing in your tank.

When you begin to purchase corals they will also be consuming ions which they use to build their skeletons and grow. Calcium, Carbonate (Alkalinity) and Magnesium are the 3 major ones. Once they have consumed all of these elements from the water they stop growing and can even begin to die.

The main reasons for changing your water in a saltwater aquarium are these:

  • Help remove Ammonia and Nitrates that build up from the ongoing Nitrogen Cycle
  • Help remove Phosphates that Algae use to thrive and overrun your beautiful Aquarium
  • Help remove rotting material and detritus before they begin to break down
  • Replenish nutrients and minerals used by your invertebrates and coral to survive and grow
  • Prevent small chemical imbalances from becoming large imbalances over time
  • Help maintain oxygen levels if you don’t have much water movement and surface agitation
  • Help maintain crystal clear water

So now I’ve informed you on the major reasons why you need to establish a regular water change schedule, let’s get into the nitty-gritty on how to carry it out!

How Often Do You Need To Do An Aquarium Water Change?

Water changes, in my opinion, are one of the most important parts to maintaining a healthy and successful aquarium. To me, figuring out how much time I could dedicate to my aquarium each week and when I can dedicate that time was one of the first things I sorted out.

You want to try and find an evening in the week you can dedicate to your aquarium maintenance. Ideally, you should want to change the water every week. But your schedule may dictate that you can only find the time to change water every 2 weeks or even only once per month.

Finding the schedule that works for you and sticking to it will provide the stability and consistency your tank needs to be able to thrive.

My Aquarium maintenance evening is Tuesday night and I try to do this without fail! Yes, life gets in the way but I try and keep it to Tuesdays as much as I can.

The more often you can change water, the less volume you have to change, which takes less time.
Little and often or Large and Seldom.

Blue Acropora

How Much Water Do You Change In An Aquarium?

How often are you able to change water?
If you change water every week a volume of 10-15% is recommended.
If you change water every 2 weeks a volume of 15-20% is recommended.
If you change water once per month a volume of 25-30% is recommended.

These are just guidelines from years of experience from aquarists all over the world. If you have a heavy bio-load you may need to change more often than if you have a light bio-load.

If your corals are consuming faster than you can replenish with water changes then you can look at supplementing but that’s for another article.

You don’t want to change too much water in one go because this could shock your tank and cause lots of problems. Slow and subtle is how you change ANYTHING in a saltwater aquarium.
The only time you would do a large water change is if you need to save your tank from a serious problem, eg: Someone spills a drink into your water

Which Kind Of Salt Do You Use In Aquariums?

Table salt right?! Nope. But it’s salt! Yes, it is but the salt mixes designed for use in your saltwater aquarium are a blend of some 30+ minerals and ions to ensure your inhabitants thrive in their home.

You can find out more on aquarium salt mixes in my ‘How To Measure Salinity In An Aquarium’ article.

By using salt made for your aquarium you will ensure your tank will flourish. stocks the most popular brands and has great shipping considering its weight:

They all come in a range of quantities so you can purchase the best size for your aquarium. A 200 gallon box of Instant Ocean would be silly for a 15 gallon nano-tank!
Your salt is susceptible to moisture once the bag or bucket is opened, so you don’t want to keep it sitting around for months on end. It will end up like concrete!

What Type Of Water Do You Use In Reef Tanks?

Just like the salt mix, the type of water you use can really affect the long term health of your aquarium. Using water that is full of Chlorine, Phosphates, Nitrates and more are asking for algae blooms and a sickness.

Which would you rather live in? Crisp, clean mountain air or a highly polluted, dusty city? I know which one I would choose! The water you add is just the same as this analogy. The better the water quality, the better your aquarium will be.

There are 4 types of water that you can use in your water change mix:

  1. Water straight from the tap/faucet
  2. Water from the tap then treated with water conditioner
  3. Bottled water bought from a store
  4. RO/DI filtered water

Tap/Faucet Water


Do you know what your city injects into your water to help keep it clean? Do you know the cleanliness of your pipes coming into your home?

These are some of the questions you have to ask yourself when you are thinking about adding tap water to your salt mix. There are dozens and dozens of pollutants allowed to be used by the water companies to keep your water clean and some of these will cause you trouble over time.

==> Here is a great article by Scientific American on the pollutants found within city water in 45 States <==

Adding city tap water or even untreated well water to your tank is not recommended as there are many inexpensive options to help turn that water into a cleaner alternative. See Below.

Water Treated With Water Conditioner


Aquarium water conditioners are a chemical solution that you add to your bucket of water and it converts many of the pollutants into safe compounds that can be removed by your filtration systems.

These solutions are great for someone new to the hobby and for those who have not yet purchased on RO/DI filtration unit for use at home. More on that later.

By far the most popular water conditioner on the market is Prime by Seachem ( link). It is inexpensive and will treat huge quantities of water with just one bottle.

Seachem Prime Aquarium Water Conditioner
Seachem Prime Aquarium Water Conditioner

It is always a good idea to have a bottle of Prime on the shelf in case you have make a lot of water quickly for an emergency water change. I always have 20 gals of fresh water and salt mix made up but I still have a bottle ready!

Bottled Water


Bottled water is a far better alternative to city/well water. Many people who own Pico and Nano tanks will buy ‘Flats’ of bottled drinking water to use for water changes and top off because they are using small quantities.

If you are going to be using larger quantities you can purchase the large 5 gal water bottles used for drinking fountains, then reuse and refill the bottle. Many grocery stores have machines which allow you to refill them and most towns have a water supplier that looks after all the business drinking stations, they will be able to refill your bottles too.

Be aware that the grocery store filters may not be kept up to date and your water may not be as clean as you think, but it is a better alternative. You should also be able to buy bottles of fresh water and ready-made salt mix at your local fish store. But this will get expensive!

RO/DI Filtered Water


Every experienced aquarium owner will have their own RO/DI water filter in their home. For a few hundred dollars it will give you perfectly clean water and you have control over when the filters are changed!

Most owners will have at least a 4-Stage system similar to This One from

Marine Depot 4 Stage Advanced RO/DI System

The 4 stages of filtration consist of:

  1. Sediment Block Filter
  2. Carbon Block Filter
  3. RO ( Reverse Osmosis ) Membrane Filter
  4. DI ( De-Ionised Resin ) Filter

These will give you water with 0 TDS ( Total Dissolved Solids ) and because of this, your water will be the cleanest it can be to ensure your inhabitants are happy and healthy.

==>To learn all you need to know about selecting the right RO/DI unit for you, please read my article HERE…<==

Mixing Issues & How To Fix Them

Mixing Water Cloudy:

Always add salt granules to cool water, not warm.
Ensure the mixing vessel is clean and rinsed thoroughly before mixing.
Salt granules added in one go, try and sprinkle into the vessel.

Undissolved Salt Granules:

Needs more time to mix.
Water not at the correct temperature and too cold. Leave to warm up

Hard Salt In Bag/Bucket:

The granules have been exposed to moisture. Dispose of all hard packed salt.

To Finish

Changing your water is one of the most laborious jobs of maintaining any aquarium, but it is one of the most important jobs. Try and find a routine to make it as simplified as you can to ensure that it does not become a hateful task that will get put off longer and longer.

Having a clean and pristine aquarium will enhance its beauty and make the tank become a new member of your family in no time. Ensuring good water quality is a sure fire way to fend off major algae outbreaks & sickness and, not only will your aquarium sparkle, but your inhabitants will be enjoying a beautiful and healthy life.

Further Reading

If you found this article helpful, may I suggest some of our other posts that you may like:


Hi, I'm Richard and I have been an avid aquarist for over 30 years with a passion for Saltwater Aquariums. I love to pass on my knowledge to help others get the same amount a pleasure out of this hobby as I do. View my About Me page to find out more about me & my mixed reef aquarium.

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