Why Do I Have High TDS After Changing Reverse Osmosis Membrane?

A reverse osmosis membrane is used to separate unwanted molecules and particles from your water. Many people rely on them to make sure the water they put in their aquariums or fish tanks is suitable. However, some people have noticed that their water’s TDS levels are much higher once they change the RO membrane. Why is this?

TDS stands for total dissolved solids. The RO membrane is supposed to separate the ions and other particles from the water, resulting in a lower TDS level. Ideally, you want a low TDS level because that means that the water has been purified. When TDS levels are on the high side, the water isn’t of good quality for drinking or filling a fish tank

Let’s take a closer look at why this happens so you can fix the issue.

Does RO Reduce TDS?

Yes, the RO is supposed to reduce the TDS levels. This is a very efficient method of treating your water (and wastewater) to reduce any unwanted solids in the water. When everything is working correctly, this method should remove anywhere from 95 to 99 % of the water’s TDS. This includes the removal of bacteria, organic matter, salt, and all solid substances.

What Should TDS Be After RO?

In this case, a lower number is ideal. The TDS levels should be a maximum of 500 mg/ L. However, a good quality RO will likely bring it down to a much lower level. Water with a higher TDS level will taste horrible to drink and won’t be suitable for your fish’s environment. 

Why Is My TDS So High?

When the TDS levels are high it is because there is a higher presence of potassium, sodium, and chlorides. With the use of a  TDS PPM meter, you should be able to test your water’s TDS levels. If they are higher than 500 mg/ L then you will need to find the root of the problem so you can get those levels down. 

If you’ve recently changed the RO membrane and are experiencing high TDS levels, there are several causes as to why this could be happening:

  • Change in water source’s quality
  • Post-filter is adding minerals to the water
  • Re-using rejected water
  • Faulty RO membrane
  • TDS creep causing incorrect testing

Once you know what the cause of the high TDS levels is, you will be able to lower them. 

Change In Water Source’s Quality

If you use municipal water, you may notice the TDS levels fluctuate every now and again. While the EPA has set a maximum level of 500 ppm for TDS, there are certain things that could cause that to change. Certain sources could cause the TDS levels to raise, such as:

  • High flow rates
  • Soil erosion
  • Urban runoff
  • Septic or wastewater overflow
  • Organic particles

When the RO membrane is in place, it should remove over 90 % of these particles and contaminants. Unfortunately, anytime the TDS levels of your water source change it will also affect the quality of your purified water.

Post-Filter Is Adding Minerals To The Water

One thing to keep in mind is that it’s common for RO systems to come with a remineralizing post-filter. This filter does add beneficial minerals to the purified water which could increase the TDS level. If your RO system includes that post-filter then it is likely the cause of the sudden TDS increase.

Re-Using Rejected Water

If you have a countertop RO system and notice that the TDS levels are higher than normal, your system could be recycling the rejected water. This is why it’s important to discard the rejected water or else it will go back into the system. When this happens, there will be higher concentrations of contaminants in the water causing the TDS levels to raise a great amount.

Faulty RO Membrane

When an RO membrane is working properly, it will filter at 0.0001 microns (or micrometers). Any particles that are larger than 0.0001 microns won’t be able to get through, only allowing substances smaller than that to pass through the membrane to keep the TDS levels low.

If you have a faulty RO membrane then it could be the cause of a higher TDS level. Another thing to consider is that the membrane could be ruptured or damaged in a certain area, which is why larger particles are getting through. If there is a high amount of chlorine present in the water or high water pressure, it could cause a rupture in the membrane.

TDS Creep Causing Incorrect Testing

Often if the RO system has been idle, the pressure in the membrane will equalize. Since this happens, when you test the water it’s important to discard the first 2 cups of water that come out of your tap. If you test the first cup of water you pour, then you will be testing water that has a presence of TDS Creep.

If the membrane has been idle, the first cup or two of water poured won’t actually show its performance. Once it has been flowing, you are likely to get a more accurate TDS level reading. 

What Is The Best TDS Level For A Fish Tank?

When it comes to filling up a fish tank or aquarium, the ideal TDS level should be in the range of 200 to 300 ppm. Anything higher than 300 is risky for the fish’s habitat. Lower than 200 ppm isn’t recommended, but it doesn’t pose as great of a risk as high TDS levels do.

If you want to dissolve salt with water for your reef tank, TDS level needs to be as low as possible, to achieve this, you can use deionization (DI) resin.

Final Thoughts: Why Do I Have High TDS After Changing RO (Reverse Osmosis) Membrane? 

Before you fill up that fish tank, it’s important to check your water’s TDS levels. If they are on the high side, the water isn’t safe to use. If you have changed the RO membrane and are experiencing high TDS levels, they are likely caused by one of the issues above.

Once you determine the cause, you should be able to lower your TDS levels to make that water safe for fish.