API pH Down Freshwater Aquarium Water Treatment, 16-oz bottle

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About This Item

Details

  • Safely lowers pH in freshwater aquariums to desirable levels.
  • Fast-acting, concentrated, colorless and easy to use.
  • Phosphate- and nitrate-free, so it should not encourage algae growth.
  • Formulated to not cloud or cause water discoloration.
  • Helps keep your freshwater aquarium water in tip-top condition for your finned friends.

Instructions

FAQ

Learn More About API From Our Team of Experts

About API Water Conditioners

It’s Smart To Be Proactive!

The best way to keep your aquarium clean is to maintain good water quality. This will help keep your fish strong and vibrant and prevent disease. Water conditioners and test kits from API can help you keep your aquarium water safe and healthy.

How Do I Maintain a Healthy Environment?

It is important to test your aquarium water weekly to ensure it stays healthy for your fish.  In addition, a bi-weekly partial water change of no more than 25% will help reduce toxic buildup, remove debris the filter missed, and replenish the oxygen available to your fish.

Feed Twice Daily and Test Water Weekly

Feed your fish twice a day, check their water temperature, and observe them to make sure they are active. You should also test water quality weekly for optimal safety. For fish to thrive, their water must closely match that of their natural environment—poor water quality can cause stress, illness and even fish loss. The 6 important parameters for healthy aquarium water are ammonia, nitrite, nitrate, pH and water hardness (GH & KH). All of these can vary over time and are invisible to the eye.

Change Water and Restore the Ecosystem

In a closed aquarium environment, there are no natural rivers or streams to carry away debris and other waste materials. That is why you should replace 25% of your aquarium water every two weeks, making sure to add water conditioner to your tap water to make it safe for your fish. You should also change your filter media and clean your gravel monthly. Adding beneficial nitrifying bacteria with API Quick Start when you do will help restore your aquarium’s fragile ecosystem and avoid disruption.

What Do API Products Do and When Do I Use Them?

API Test Kits

Use test kits weekly to monitor water quality parameters, which can vary over time and impact the health of your fish.

API Stress Zyme

Using this every week will help keep your aquarium clean and your gravel and ornaments sludge-free.

API Algaefix

Use weekly to control algae growth and keep aquarium glass, ornaments and plants clean.

API Stress Coat

Use every 2 weeks during water changes to make tap water safe, and when adding fish, or when fish are injured.

About API

The team at API has a passion for fish and understands the rewards and relaxation fishkeeping can bring. For almost 60 years, they have developed premium products with a focus on proven and effective results. API offers a range of testing kits, water conditioners and nutritionally superior fish foods. They are dedicated to making a better underwater world for the benefit of fish and those who keep them.

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Questions & Answers

9 Customer Questions

I am confused. Does pH down make the water more alkaline or acidic?

Answer by Chewy • Mar 13, 2018
API pH Down Aquarium Water Treatment will safely and quickly lower the pH in your freshwater aquarium for more acidic water conditions.

So i misread the directions and added too much. Now my ph is too low. Can i add the Ph up to correct it? I don't have fish yet, I'm prepping my tank for them.

Answer by Chewy • May 05, 2021
We would recommend to perform a partial water exchange and retest the water.

How much should i use for a 10 us-gallon tank?

Answer by Chewy • Nov 05, 2019
To lower pH, add ½ teaspoon (2.5 milliliters) for each 25 U.S. gallons of aquarium water. A 10 U.S. gallon tank would require about 1 milliliter.

Is it safe with dwarf shrimp?

Answer by Chewy • Sep 04, 2019
This treatment is safe for invertebrates (such as dwarf shrimp), when dosed as directed.
Reviews
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4.6
Rated 4.6 out of 5 stars
80 Ratings
Terms & Conditions

21 Customer Reviews

Rated 1 out of 5 stars
By Rebekah on Nov 10, 2025

Didn’t Help

I’ve tried this several times according to the directions and it did not change the PH in my fish tank.

I’ve tried this several times according to the directions and it did not change the PH in my fish tank.

Rated 5 out of 5 stars
By david on Oct 18, 2025

Water fixer

It lowered my PH to where I needed it to be. Simple instructions and pretty self explanatory.

It lowered my PH to where I needed it to be. Simple instructions and pretty self explanatory.

Rated 5 out of 5 stars
By Brooks on Mar 20, 2025

Instant

It lowered quickly didn't need to keep trying to lower the ph love it

It lowered quickly didn't need to keep trying to lower the ph love it

Rated 5 out of 5 stars
By Louis on Feb 27, 2025

Great Service

The items were great. The delivery was quick, and the prices were good

The items were great. The delivery was quick, and the prices were good

Rated 5 out of 5 stars
By Dan on Feb 2, 2025

This product is easy to use

Along with test kit it keeps the water parameters balanced

Along with test kit it keeps the water parameters balanced

Rated 5 out of 5 stars
By Christine on Aug 21, 2024

Thankful!

This helped bring down the ph in my goldfish tank! Don't use a lot at once, it can shock fish if it comes down too much overnight. As long as you know this, it'll

This helped bring down the ph in my goldfish tank! Don't use a lot at once, it can shock fish if it comes down too much overnight. As long as you know this, it'll work great!

Rated 5 out of 5 stars
By Dawn on Jan 31, 2024

Dealing with PH

I own a Mata Mata turtle & it is extremely important for the aquarium water to have a very low PH level. This liquid treatment helps!

I own a Mata Mata turtle & it is extremely important for the aquarium water to have a very low PH level. This liquid treatment helps!

Rated 5 out of 5 stars
By Andrzej on Mar 11, 2024

API ph Down

Arrived quick and works great brought be down from ph 8.2 to 7.4

Arrived quick and works great brought be down from ph 8.2 to 7.4

Rated 5 out of 5 stars
By Scot on May 15, 2024

Great product

Even better on sale. Thank you.

Even better on sale. Thank you.

Rated 5 out of 5 stars
By Jessica on Jul 27, 2023
Top 500 Contributor

love the size

This big bottle is way better than the sizes at my local pet store and it works !

This big bottle is way better than the sizes at my local pet store and it works !