How To Lower TDS in Aquarium (Complete guide + tips)

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by Jason Matthews



You must continuously monitor many water parameters to maintain the optimal health of your aquarium. PH, KH, GH, TSS, and TDs are all important measures. TDS stands for Total Dissolved Solids. It refers to the amount of dissolved particulate in your tank’s water. 

The top recommended ways to lower the TDS in an aquarium are using RO (reverse osmosis) water, deionized water or rainwater, changing the water regularly, and practicing controlled feeding.

The TDS level in your tank’s water could be high for a variety of reasons. Read on to learn what causes high TDS levels, and what you can do to lower them.

Causes of High TDS

It’s important to understand that particulate matter in the water does not necessarily indicate that something is wrong with your tank. The particulate matter could come from salt or minerals or could come from fish waste, fertilizers, de-chlorinator, and even food.

Anything you put in your fish tank can contribute to TDS. Any particulate matter that is small enough to fit through the filter will count toward your tank’s total TDS level.

Naturally, the level of TDS in an aquarium will rise over time. Daily feeding, de-chlorination, and fishes’ biological need to eliminate waste all contribute to rising TDS. Simple evaporation can contribute to higher TDS levels. As water evaporates, solid matter is left behind, meaning the concentration of particulates left in the tank will be higher. 

Why is Maintaining the TDS Level Important?

In general, fish like their environment to stay consistent. Large fluctuations in temperature, PH, TDS or other parameters can cause stress to the life inside your aquarium. 

TDS affects the passage of water in and out of the bodies of some marine life, like shrimp. High levels of TDS affect the process of osmoregulation or how fish and other marine life balance their fluid, electrolyte, and mineral levels.

As TDS levels rise, aquatic creatures’ bodies attempt to equalize with the surrounding water. This equalization can lead to problems like salt stress and can exacerbate the buildup of minerals like calcium and magnesium in their bodies, which leads to breeding problems.

High concentrations of solids in water also affect water clarity. At extremely high levels, clarity can become so poor that plants in the tank have trouble performing photosynthesis.

How to Lower TDS in an Aquarium

The four main ways of lowering the TDS in your aquarium are to use RO (reverse osmosis) water, use deionized water or rainwater, change the water regularly, and do controlled feeding.

Use Reverse Osmosis Water

Reverse Osmosis, or RO, water is purified using a semi-permeable membrane to filter out molecules and particles like contaminants, salt, minerals, dirt, and chlorine.

Adding RO water to your tank will dilute the particulate matter in the aquarium because RO water has a naturally low particulate level. You may even need to add some essential minerals to the RO water before adding it to the tank to ensure that the TDS level does not get too low.

If you do not want to add minerals to the RO water before you use it to dilute your tank, only change about 5-10% of the water at a time to ensure the mineral balance does not get too low.

Another way of using RO water in your tank is to install an RO unit. An RO unit that filters your tank’s water through a semi-permeable membrane can remove up to 95-98% of particulate matter from your tank. 

Use Deionized Water or Rainwater

rain water for aquarium

Deionized water is another form of purified water. The purification happens through deionization, or by removing atoms, ions, and molecules through an ion exchange process. Deionized water is considered the purest form of water available—it is considered by some to be “synthetic water.”

You can use deionized water to dilute the particulate matter in your tank. You must first filter deionized water with an RO system to eliminate any non-ionic organic compounds in the water. Add deionized water to your tank as you would RO water to lower the TDS in your aquarium.

Rainwater is a good source of low TDS water because it has fallen directly from the sky and has not yet come into contact with solids and contaminants. Depending on where you live, you may be able to collect rainwater and use it to lower the TDS in your aquarium by diluting the water in your tank.

Related article: How to increase CO2 in aquariums

Change the Water Regularly

change water regularly

It should go without saying that you should change the water in your aquarium regularly. How often you do so will depend on your unique situation—where your tank is located, the types of aquatic life you have in there, how much sunlight it gets, etc.

When you notice TDS levels getting near the upper limit of what is healthy for your fish, do a 5-10% water change. If the TDS levels do not lower, attempt another 5-10% change in two or three days. Never try to change all the water at once. Approach water changes slowly and gradually.

Practice Controlled Feeding

controlled fish feeding

One of the main contributing factors to high TDS levels is food particles. Overfeeding is a common cause of many water discoloration ailments. Do not overfeed your fish. Most fish require 16-24 hours to fully digest their food, so feeding once a day is quite sufficient.

Investing in an automatic fish feeder can be a great way to keep your fish fed on schedule and avoid accidental overfeeding. These days, you can operate many automatic feeders from your smartphone, which is useful if you need to feed your fish while you are at work or on vacation.

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Wrap Up

Depending on how high the TDS level in the aquarium is, you may want to try a combination of solutions. We recommend beginning with controlled feeding, as overfeeding is often the cause of water-related concerns in home aquariums.

If you have tried controlled feeding and are still seeing high TDS levels in the aquarium, try a 5-10% water change using RO water and continue from there. 


Do aquarium filters remove TDS?

Aquarium filters do not remove TDS (Total Dissolved Solids) as they primarily target solid particles and impurities in the water. To reduce TDS, consider using a reverse osmosis (RO) system or performing regular water changes.

Can activated carbon remove TDS?

Activated carbon filters can remove some dissolved compounds but are not effective at reducing TDS significantly.

What is the ideal TDS for a fish tank?

The ideal TDS for a fish tank varies depending on the species being kept. Generally, TDS values between 300 and 450 ppm are suitable for most freshwater fish. However, some species require higher or lower levels, so always research the specific requirements of your fish.


Jason Matthews

My name is Jason Matthews, and welcome to my website. When other kids were bragging about how their dog could sit and roll over, I was bragging about my latest Betta Fish and the cool sea castle I just added to his aquarium. 

Jason aquariume

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