The standards for tap water are made so that any water running into a home will be safe for consumption by humans, but this does not mean that it will be safe for a reef tank. Most tap water still has a high level of total dissolved solids (TDS), and these ions can be harmful to your reef tank.
An acceptable TDS level for any water added to a reef tank sits between 0 and 5 ppm. The idea is to get as close to 0 as possible.
The higher the number of TDS, the greater the chances are that issues will arise in your reef tank. A higher level of TDS can feed algae and harmful bacteria, inhibit the photosynthesis process, and negatively impact the immunity of your fish.
Is 1 TDS Bad for a Reef Tank?
A TDS reading of 1 ppm is not necessarily bad for a tank, but it is not preferred. Many will suggest you only use water with a TDS measure of 0 ppm.
As long as you keep TDS under 5 ppm, you should be okay adding it to your reef tank. If the TDS of water still reads over 5 ppm after running it through your RO unit, you may need to install a D-D DI pod to completely remove anions and cations from the water.
While most tap water starts well over 100 ppm, anything over 5 ppm will be too much for the fish and plant life in your reef tank.
What is the TDS of Saltwater?
The TDS of saltwater will be high because of the salinity of the water. Saltwater usually has a TDS of at least 10,000 mg/L, and some conditions can go up to 35,000 mg/L or higher.
Saltwater, both in and outside a reef tank, will also have a higher value of TSS, or total suspended solids. These are material things, such as plant water or fish waste. In the wild, these materials are dealt with naturally, but in a reef tank, the aquarium filter catches them.
While saltwater has a high TDS, this does not mean that you can add water with high TDS to your reef tank. The difference is that the dissolved solids removed during reverse osmosis contribute to poor water conditions. These include:
- Calcium
- Sodium
- Magnesium
- Phosphate
- Nitrate
- Silicate
When you get in a saltwater tank, these materials will still be present, but they should only occur at a level the tank’s nitrogen cycle can handle. The majority of the TDS of saltwater should come from the dissolution of salt and other materials, not from the introduction of untreated water.
Is High TDS Bad for Fish?
High TDS is bad for fish. It throws off the balance of their ecosystem, reduces water clarity, interferes with the photosynthesis of plants in their ecosystem, and can even increase the water temperature.
High TDS is harmful to the entire ecosystem, but fish will bear the brunt of these effects. This is because fish are largely composed of water, and water with high TDS will want to equalize with the lower TDS inside the fish.
In an aquarium, the water will attempt to pass through the fish until these levels equal out. This can cause changes in the fish:
- pH
- Digestion
- Immune system
Fish dealing with an improperly high TDS will be at a greater risk of disease, and they are more sensitive to the imbalances in your reef tank. High TDS can leave them feeling off-balance every day. If the problem is not resolved, it can even lead to the death of the fish in the tank.
High TDS is often an indicator of another issue, especially when you are adding new water to the tank. This new water should not have a high TDS reading (if any at all), and this usually indicates an issue with your reverse osmosis system.
Water coming out of a RO system should not have a high TDS reading. These systems are designed for extreme efficiency when removing unwanted ions from water.
High TDS may indicate that ammonia and silicate are still making their way through your RO unit, or that the RO unit is on its way out. Testing the efficiency of the system is essential for pinpointing the problem.
You may find that it is time to replace the thin film membrane (TFM) on your RO unit, or that your initial water conditions merit the use of DI resin.
How Do You Measure TDS in a Reef Tank?
You should measure the TDS for a reef tank using a TDS or Electrical Conductivity (EC) meter. Measure the water before adding it to your tank. Because of the saltwater in a reef tank, it is important to measure the TDS before the water is added to the tank.
When using a TDS meter, make sure you do not touch or damage the electrodes. You can keep them clean by rinsing them in RO water. Vinegar should be sufficient to remove any scale buildup, but rinsing with RO water after is essential to get a proper reading.
Make sure that the collection vessel you use for the water is clean before testing the water. This should also be well rinsed with RO water; even small amounts of soap or detergent can lead to false high readings.
Measuring TDS is simple enough.
Remove the cover from the bottom of the meter and turn it on. The screen should read 000 by default.
When you are reading, insert the tip of the meter into the water you are testing. You should only need to go about a half-inch deep for an accurate reading. Submerging the meter too far can damage the device.
The meter will run a small electrical current through the water and use the response to determine the total dissolved solids in the water. Water has practically no conductivity on its own, and the TDS is what allows electricity to flow through it so easily. The ease at which the current travels will tell the meter what the condition of the water is.
This number should be reported in either mg/L or parts per million (PPM). Once you have this number, you will know what the TDS of the water is and whether it is safe enough to add to your reef tank.
