Is It Ever Safe to Do a 90% Water Change?

If you have any experience with aquariums, some people think they have to change their water every week or whatever they read online or what someone tells them.

Really you only need to do a water change when your nitrates get to 30-40 ppm and then you want to change enough water to get them lower.

I’ll explain how to figure this out later because first, I want to answer the people that have been asking if it is ever safe to do a 90% water change?…

It is safe to do a 90% water change as long as the new water is dechlorinated, has the same temperature, and has the same pH as the water you replaced to keep from shocking your fish. You should lead up to a 90% water change with smaller water changes first.

There’s a lot more to it than this so let’s dive in(no pun intended)…

90% Water Change For A Crowded Tank

A good example of this is someone I saw who had a crowded tank of fish that were happy together but they were producing so much ammonia that he had to do a 90% water change once a week just to keep his nitrates down at a normal level.

A normal level would be 5-10ppm or as close to zero as possible for reef tanks.

To figure this out, you would test your water for nitrates each week and as soon as they got to 30-40ppm you would need to change different amounts of water until your nitrate level got back below 10ppm.

Obviously for this guy, since his tank was so crowded, his nitrate level got too high after one week.

If the fish are happy, you can change the water once a week if you don’t want to go through the trouble of getting a bigger tank.

90% Water Changes For Certain Fish Species/Tank

Goldfish are a good example here.

Goldfish digest food through their intestine, since they don’t have a stomach, they poop a lot and are messy fish.

So, whereas some fish would only need a 20% water change per month you goldfish are going to need weekly water changes and the amount depends on your nitrate testing.

It also depends on how much space your fish has to poop as referenced above by the crowded tank.

The less space for the ammonia from the poop to dissolve, the more water changes you are going to need.

90% Water Changes For Certain Fish Diseases

Some fish diseases will need a 90% water change.

I like to use examples so a good example here would be ich which is a parasite.

If you do a 90% water change then you eliminate 90% of the parasites swimming in the water and then use medicine to target the remaining 10%.

How To Safely Do A 90% Water Change

The best way to do a 90% water change regularly in your aquarium is to do smaller water changes first so that your fish can slowly get used to larger amounts of water being changed.

Of course, in the case of a disease, you won’t have time.

So, if you have to do an emergency change or you have built up to one here we go…

  1. Test your aquarium water for nitrates and pH.
  2. If it’s time to gravel vac then do that first(also remove and clean decorations).
  3. Use a water pump to remove 90% of the water.
  4. Condition the new water with a de-chlorinator in buckets or add the dechlorinate to tap water as you’re dumping it into the tank but make sure the new water is the same temp as the water you took out. You may need to use your aquarium heater or have a spare one on hand.
  5. If your tap water doesn’t have the proper pH for your fish then you can buy buffers or adjusters online to add to your water each time that will attain the proper pH.
  6. You can refill your tank with a water pump or you can use an aquarium Python to remove and add water to your tank.
  7. Test your new water after it is in the tank to make sure nitrates are down and the pH is where you want it to be.

Voila! It’s that easy, your fish will be fine because you are putting the same type of water that you took out so they won’t be in shock.

You also can clean your filters every month – 2 months. Just make sure you clean them in tank water so you don’t lose the beneficial bacteria.


So remember fishy folks…

You can do a 90% water change for certain circumstances, just make sure you do it properly so you don’t shock your fish.


Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top