Reef tank emergencies can take different shapes, but more often than not, they can be solved with fresh saltwater. That's why having an RO/DI system and being prepared are critical in those moments where each passing minute can be the difference between saving your tank and a heartbreaking disaster.

Here are 5 things every reefer should do to make sure your RO/DI setup is ready when it matters most:

1. Store Enough RO/DI Water

Recommendation: Keep enough RO/DI water on hand to make a meaningful difference in an emergency.

Instead of keeping a few gallons on hand for top offs and routine maintenance, consider keeping a larger amount of RO/DI water ready to go, just in case. Because RO/DI systems produce water rather slowly, this can save you valuable hours in a reef tank emergency situation.

Instead of recommending a specific number of gallons to keep on hand, we suggest calculating your system's total water volume (display tank + sump) and using that to determine how much RO/DI water to store.

For example, if you have a 100 gallon system water volume, you may want to keep 30 gallons of RO/DI water at the ready so that you can do a 30% water change if it's ever needed.

2. Keep Premixed Saltwater Ready

Having RO/DI water on hand is a good first step, but if the emergency necessitates doing a large water change, you'll need to mix that water with salt and potentially heat it before it can be used.

Recommendation: Keep some pre-mixed saltwater at the ready. Even having 10, 20, or 30 gallons of saltwater ready can buy you some time while you mix up a bigger batch.

Storing Premixed Saltwater

• Use a clean, aquarium-safe container such as a Brute trash can or NSF certified water storage tank.
• Keep the container covered to prevent contamination
• Verify temperature and salinity before use
• Many salt mixes benefit from a small circulation pump to keep the water moving

Key Takeaway: Stored RO/DI water provides capacity, while premixed saltwater provides speed.

3. Maintain Your RO/DI System

A reefing emergency is the worst time to discover exhausted DI resin or other RO/DI system issues related to system maintenance you've been putting off until "next week."

Recommendation: Keep up on RO/DI filter changes, monitor system pressure, and always keep a spare set of filters on hand (especially DI resin).

Monitor Water Quality

Monitor the TDS of the water leaving the system to ensure the water you're producing isn't going to do more harm than good.

Inspect and Replace Filters

Sediment Filter
Removes larger particles such as dirt, rust, sand, and other debris from the incoming water supply. This first stage helps protect and extend the life of downstream filters like the RO membrane.

As the sediment filter begins to clog with debris like rust, sand, and dirt, the pressure entering the RO membrane will start to drop. If your system has a pressure gauge, it's time to replace the sediment filter when you start seeing a decrease in pressure. If your system doesn't have a gauge, we recommend proactively changing out your sediment filter every 6-12 months or so.

Carbon Block(s)
This stage helps protect the RO membrane from damage while also reducing chemical contaminants. The dual carbon block configuration is ideal for removing both chlorine and chloramines, a common disinfectant used in city water sources.

Carbon filters are harder to monitor directly, so they are usually replaced on a schedule. Most hobbyists change them every 6 to 12 months. If your water supply contains chloramines or you produce large amounts of RO/DI water, replacing them closer to every 6 months is recommended.

RO Membrane
Acts as the core filter of the system by removing the majority of dissolved contaminants (TDS) from the water. Under pressure, purified water passes through the membrane while impurities are rejected into a separate waste-water tube. The water is then filtered through a second membrane for faster water production and less waste water.

The RO membrane typically lasts around 3 years when the sediment and carbon filters are replaced regularly. You can also monitor performance by comparing the TDS of the incoming water to the TDS after the membrane. Most membranes reject about 98% of dissolved solids. For example, if the incoming water measures 100 TDS, the water leaving the membrane should be close to 2 TDS.

DI Resin
Deionization resin is the final stage of the RO/DI system and will polish your water down to 0 TDS. The DI resin is most often a mixed bed with a color changing indicator to let you know when it is becoming depleted. The resin starts as a dark blue color that will turn gold as it becomes exhausted.

If the water leaving the system reads above 0 TDS, the resin is exhausted and should be replaced. If you do not have a TDS meter, color-changing DI resin provides a visual indicator. Replace the resin just before the color change reaches the top of the cartridge.

4. Keep the Complete Water-Change Setup Available

Having RO/DI water and salt mix on hand is great, but if you don't have the right tools to mix it, test it, or transfer it to the aquarium, it's like not having it at all.

Recommendation: Keep dedicated saltwater mixing supplies on hand at all times.

Essential Water-Change Supplies

• Clean, 0 TDS RO/DI water
• Enough salt mix for a meaningful water change
• Clean mixing container
• Circulation pump
• Salinity testing equipment (refractometer, digital tester, etc.)
• Transfer pump & clean tubing
• Heater (sometimes optional, but usually recommended when doing large water changes)

Temporary Holding Tank

In the event of a major emergency, moving your livestock to a temporary tank may be the best choice.

In addition to the supplies above, consider keeping a few buckets or an inexpensive 10 to 20 gallon aquarium on hand, depending on the size and number of fish and coral in your system.

5. Create a Water Resupply Plan

Stored water can help you respond immediately, but some reef tank emergencies require multiple water changes or continued water production over the next day or two.

Recommendation: Know how quickly your RO/DI system can replace the water you use and identify a backup source before an emergency happens.

Know Your Actual Production Rate

The gallons-per-day rating listed on an RO/DI system represents how much water it can produce over a full 24-hour period under ideal conditions. It does not mean that amount of water will be available immediately.

Real-world production may also be lower than the advertised rating. Cold source water, low water pressure, clogged prefilters, aging filters, and declining RO membrane performance can all slow down water production.

Pay attention to how long your system actually takes to fill its storage container. Once your stored water is gone, that production rate will determine how quickly you can prepare the next batch.

Plan for Additional Water

• Keep additional clean, aquarium-safe storage containers available.
• Know approximately how long your RO/DI system takes to refill your reservoir.
• Identify a nearby local fish store that sells RO/DI or premixed saltwater.
• Talk with other local reef keepers who may be able to help in an emergency.
• Know where you can purchase distilled water as a temporary backup if suitable aquarium water is unavailable.