Benefits of Changing More Than 50% of Your Aquarium Water?

4 min read

It can be so exciting to get your first tank set up and cycled and add your favorite colorful fish that you’ve been wanting, but what if something goes wrong?

Keeping a fish isn’t so much about keeping a pet as it is about keeping clean water in your aquarium.

Instead of being called a fish keeper, you should be called a water keeper.

So, we get a lot of questions about water changes, how often to do them, and how much water.

One of the questions I’ve been asked recently is, can I change more than 50 percent of the water in my aquarium?


You can change more than 50% of the water in your aquarium but only to keep nitrate levels down for certain fish species, a crowded tank, or certain fish diseases. You can only change more than 50% of your aquarium water as long as you put water with the same chemistry and temp back into the tank.

What Level Do My Nitrates Need To Be At?


People are always arguing in the aquarium online world about the highest allowable nitrate level for aquarium fish but there is not a factual answer written in stone.

Aquarium fish can tolerate very high nitrate levels but a good number to aim for is 40 ppm(parts per million).

There is plenty of scientific evidence to support the theory that fish will survive above 40 ppm but it is a good number to aim for.

How Often Do I Need To Change My Water To Keep The Nitrate Level Below 40ppm?


This depends on many different factors:

  • Species
  • Tank Size
  • Fish Size
  • Quantity

After your aquarium is cycled you will have to test the water every couple of days or even every day if you want to.

The idea is to change the water as soon as it gets close to 40 ppm of nitrates.

You can keep changing 25% of the water until you get the nitrates to the level that you want.

Species – Sometimes, for example, if you have a goldfish in a small aquarium, you may need to change 50% of the water each week to keep your nitrates down.

Goldfish poop more than most fish so you get the point. Some fish don’t have that much poop so you may only need to change 25% of the water every two weeks.

Tank Size – If you have a bigger tank with fewer fish then you won’t have to change as much of your water as a smaller tank with more fish.

I’ve seen a person with an overstocked tank change 80% of their water each week and the fish were fine because they had clean water and were used to it.

Fish Size – Again, it all depends on how much your fish poops in and how much space they have. If you have a four-inch fish in a twenty-gallon tank, the water will need to be changed more than if it were a one-inch fish in a twenty-gallon tank.

Quantity – Again, if you have two fish in a twenty-gallon tank, the water won’t need to be changed as much as four fish in a twenty-gallon tank.

How To Safely Change More Than 50% Of Your Water in your Aquarium.

The best way to do large water changes regularly is to start out with smaller water changes and gradually build up to a larger water change.

Regardless, when you change the water the most important things are to keep the new water the same temperature, dechlorinated, and the same pH as the water you take out so you don’t shock the fish.

Water TemperatureThe easiest way to keep the water at the same temperature is to adjust it coming out of the tap so you don’t have to heat the water.

Chlorine – You should always add water conditioner to your water to take the chlorine out. Chlorine kills bacteria and you have to have some beneficial bacteria in your tank. Our favorite water conditioner is prime.

Same pH – you may not need to do anything to your water to keep it at the same pH. If you need to adjust the pH of your water, there are several products you can buy to add to your water.


I hope we have shown that it isn’t a big deal how much water you change as long as you do it properly to keep from shocking your fish.

Just change as much water as you need to keep the nitrates down.

Start with little changes first if you need to lead up to a big one.

Make sure the water is the same temp, pH, and dechlorinated.


Written by:

Pet Aquariums

Have you any questions?