How to Refill a DI Resin Cartridge in an RO/DI System

In a saltwater aquarium, water quality is everything. That is where your RO/DI system comes in. The final stage of this filtration setup is the deionization (DI) resin, which removes the last traces of impurities left behind after the water has passed through all the other filters. This polishing step ensures you are starting with ultra-pure, zero TDS water, which is essential for mixing saltwater and keeping corals, fish, and invertebrates healthy. Over time, the DI resin becomes exhausted and must be replaced.

How to Refill the DI Resin in a Refillable Cartridge

1) Inspect for Damage, Then Unscrew the Cap

Remove the cap from the DI cartridge. Inside, you'll see a foam ring. Take this out and set it aside for later if you plan to re-use it. Inspect the cartridge for any cracks and replace it, if necessary.

Tip: It's a good idea to replace the foam ring when you refill your cartridge as it will get compressed over time. At a minimum, replace it every few refills.

2) Dump Out the Old DI Resin

If you're refilling a used cartridge, dump out as much of the old resin as you can into a trash bag. Rinse the cartridge to get rid of any left over DI resin beads.

3) Add the New DI Resin

Cut a hole in the corner of the DI Resin Bag. Slowly pour the new DI resin into the cartridge. Stop when the cartridge is about 3/4 full and give the cartridge 10 solid taps on your work surface to pack the resin down as tightly as possible. The tighter the resin is packed, the more effective it will be.

Tip: Use a canning funnel to help guide the DI resin into the cartridge and cut down on spills.

4) Top It Off & Pack It Down

Keep adding resin until the cartridge is completely full and level with the top. Make sure you cannot fit any more resin inside.

Note: If you're using a single-use BRS brand resin refill bag, it's normal to have a little resin left over.

Tip: A BRS 250 mL Measuring Cup works great for giving the resin one last firm press into place.

5) Make a Shallow Groove Around the Rim

Use your finger to make a small grove around the rim of the cartridge. This ensures there is room for the foam insert and cap so you don't end up with beads squishing out into the threads as you put the cap back on.

6) Replace the Foam Ring & Screw on the Cap

Make sure the threads on the DI cartridge are clear from any stray DI resin beads. Put the foam ring back inside the screw cap and secure the cap onto the cartridge.

Tip: Apply even pressure to the cap with the palm of your hand and turn the cartridge itself to secure the cap.

7) That's It! Now Just Reinstall the Cartridge

Insert the cartridge back into your RO/DI system's DI stage canister. Check that the arrow on the cartridge is pointing up. 

DI Resin Troubleshooting & Common Questions

My TDS Meter is Showing Higher TDS After the DI Stage

Common Cause #1: The DI cartridge is installed upside down. Check to make sure the arrow on your cartridge is pointing up. The rubber seal should be at the top and the cap should be at the bottom.

Common Cause #2: The DI resin wasn't packed tightly enough. If water can "channel" through the DI resin, the resin will not be effective. This can lead to poor performance or higher TDS exiting the resin.

Common Cause #3: Running BRS PRO series resin and measuring TDS after the Cation stage. We won't get into the science behind it in this guide, but it's normal and expected for the TDS reading to read higher after the cation stage. You must also run a separate Anion resin stage afterwards and cannot use the Cation resin alone.

My DI Resin Smells Terrible!

If you're noticing a strong smell from your DI resin, you're likely using the PRO series Anion resin. It's normal for this resin to have an ammonia-like scent. If you or someone else in your household is sensitive to smells, it may be best to pack this resin outdoors or in the garage.

How Often Should I Replace the DI Resin?

The unsatisfying answer is...it depends. How often you'll need to replace the resin depends on factors like how much water you're producing, the quality of the source water, how effectively your RO membrane is performing, and more.

As a general rule of thumb, we suggest changing out the DI resin when the color change is about 1" from the top. If you're running dual DI resin canisters, you can fully exhaust the first cartridge and then rotate them in a first in, first out fashion.

I Am Going Through DI Resin Incredibly Quickly; is There a Fix?

Rapid DI consumption can be caused by a few things.

Common Cause #1: High CO2 in your source water. CO2 will pass through your RO system's pre-filters and RO membrane with ease. This isn't necessarily a water quality concern, but it can lead to faster DI resin consumption and is most common when the source water is a private well.

Common Cause #2: Frequent on/off cycles. As the RO/DI system sits, the water in the membrane housing will equalize on both sides. This means that when the system is first turned on, there is a small amount of higher TDS water that enters into the DI stage(s). For that reason, we recommend using the RO/DI system for longer periods of time to make a lot of water at once instead of many short on/off uses to produce the same amount of water. 

So what's the fix? 

The easiest fix is to use a triple DI setup with our PRO series resin. The reason why as well as some more in-depth information is available in the video linked below.

PRO Series Resin Video: Upgraded RODI water filtration w/ BRS Pro Series DI Resin

This Seems Like Too Much Work. Is There a Pre-Packed DI Resin Option?

Yes! We offer standard color-changing DI resin that is pre-packed in a cartridge for easy and mess-free DI resin changes.

Available Here: Pre-Packed Color-Changing DI Resin Cartridge

I Still Need Help!

If you have any other questions or need further help with your RO/DI system, call our reefing experts at 763-432-9691 or send an email to support@bulkreefsupply.com.