Marine tank

Reverse osmosis filter in marine aquaristics (RO Di)

What does it mean for the water to be pure and is a reverse osmosis filter (RO Di) a necessary equipment in marine aquaristics? To answer these questions, let’s first focus on the purity of tap water. One might assume that drinkable water is clean. On the other hand, however, we often hear people saying that their tap water is unpalatable, hard, has a brownish color or simply smells of chlorine. This means that there must be some chemical compounds in this supposedly pure water.

What are the parameters of tap water?

The quality of drinking water is supervised by water treatment plants that make sure it meets purity standards. They specify dozens of parameters that must be met. Below you can find those that are of greatest importance to marine aquarists.

European standards for certain chemical compounds in drinking water
*not directly regulated  
**for phosphorus 2.2 mg/l

As for calcium and magnesium, their content is regulated by the permissible level of water hardness. Water hardness is affected by many ions, but the standard is given in terms of calcium carbonate. In Poland, the hardness of water approved for consumption must be in the range of 60-500 mg/l CaCO3. If we convert this value to calcium, we will get an acceptable calcium level in the range of 24-200 mg/L.

As for the aforementioned standards, I must point out that these are the maximum permissible limits. In most cases, the recorded contents of these elements reach about half of these values. This means one thing: as long as all the values (including those I haven’t cited here, which can be found on the European Commission’s website) are within the acceptable range, no actions will be taken.

However, two things should be kept in mind. First, the quality of tap water is subject to seasonal fluctuations. It can also vary significantly depending on the location of the water intake. Second, water monitoring usually takes place at the outlet of the treatment plant, not in the customer’s water supply system. Water quality can be affected by the length of transportation, storage, the condition of the water supply network or the speed of flow. As a result of corrosion of older plumbing systems, zinc, iron, copper and lead compounds seep into the water.

Is tap water good for a reef aquarium?

The use of tap water in marine aquaristics may cause the risk of introducing substances toxic to marine invertebrates, such as copper compounds, into the aquarium. The copper content of drinking water is regulated by the relevant European Commission directive. It says that he allowed amount of copper is 2 mg/l. This is a safe value for humans, but for corals or shrimp it is lethal. Copper ions Cu2+ are toxic to marine organisms even at low concentrations. Values close to 0.04 mg/l already cause some corals to bleach after 48 hours, although studies have shown that some corals can withstand much higher concentrations of copper. Hypothetically speaking, the extreme allowable copper content in tap water (2 mg/l) is about 220 times higher than the concentration of copper in the seas (0.009 mg/l) and about 50 times the lethal dose for some corals.

But there’s one simple and relatively inexpensive way to deal with all the unwanted substances in the water. It is a reverse osmosis (RO Di) filter.

How is a reverse osmosis (RO Di) filter constructed?

RO Di filter is a device comprised of several stages of filtration. The first two or three elements are so-called prefilters. These are mainly 1, 5 or 10 µm sedimentation filters and an activated carbon filter. Their taks is to retain suspended solids and chemical compounds, such as chloramine, which have a destructive effect on the most important part of such a filter – the osmotic membrane.

This membrane in simple words is an microscopic mesh screen. Tits wholes are so small that they let water molecules pass through, and all the larger ones are filtered out and removed from the water.

What do you need to know about reverse osmosis filter?

The membrane and prefilters wear out and need to be replaced. While a well-maintained membrane can serve us for several years, prefilters need to be replaced every few months, especially if we produce a large amount of water for the aquarium.

In addition, the membrane needs adequate pressure in the water supply system (80-100 psi) to work efficiently. In most homes, the pressure is insufficient, which reduces the filter’s effectiveness. The solution is to install a booster pump that raises the pressure on the membrane.

It is worth remembering that the membrane purifies water at about 97%. This means that 3% of the contaminants pass through the filter. An effective way to deal with these impurities is to use a so-called deionizing resin (Di) that absorbs all ions that have passed through the membrane. Of course, such resin should also be replaced regularly.

Unfortunately, in order to obtain 1 liter of RO (i.e. filtered) water, we need to use about 5 liters of tap water, since the membrane must be constantly rinsed with water. In this respect, it is not a very efficient device, and it can turn out to very expensive as well. There are methods to improve the performance of the filter, but they require a few modifications.

Reverse osmosis filter is essential for brine preparation

One way to take care of the water quality in a marine aquarium is to change the brine regularly. The aquarist drains, for example, 50 liters of water from the aquarium and replaces it with freshly prepared brine. The whole process is very simple, but is of great importance in maintaining optimal water parameters.

Take, for example, Tropical Reef Base Salt. If dissolved at the right salinity (usually 33-35 ppt), it gives water parameters similar to natural seawater.

Tropical Reef Base Salt

As I’ve mentioned before, the composition of tap water can vary greatly depending on the season or location. This means that dissolving salt in tap water from two different intakes will yield brines with different parameters. Such a situation not only prevents good results in aquaristics, but also makes it difficult to deal with any problems . When we dissolve salt in RO Di water, the resulting brine receives the parameters planned by the manufacturer, because there is nothing in RO Di water that disturbs the balance of the components. Using Tropical Reef Base salt, you get water with parameters similar to natural seawater, as planned by the manufacturer.

In the diagram above, all the bars balance around the red line, indicating seawater parameters. Fish and marine invertebrates will thrive in an aquarium where the water holds natural parameters. Now let’s see what can happen when salt is dissolved in tap water.

Unfiltered water contains various dissolved substances. If the calcium level measured in the tap water is more than 250 mg/L, this amount of calcium will accumulate with the calcium from the salt used. As a result, the calcium level in the brine will reach more than 650 mg/L. This amount of calcium in the water is dangerous for marine animals.

Should I use RO Di filter in marine aquaristics?

The water in every tap has a different chemical composition, and the RO Di filter “resets” this composition to pure H2O, so that RO Di water anywhere in the world is almost identical, and in this case “almost” makes no difference.

Remember that taking care of your home reef basically means taking care of water parameters such as pH, KH, calcium and magnesium. The problem is that while it’s easy to bump them up when they’re too low, it’s hard to reduce them when they’re too high. Therefore, the use of well-balanced salt along with supplements and RO Di water is essential for obtaining and maintaining proper water parameters in a marine aquarium.

Marine aquaristics is a challenging hobby. Some challenges are a pleasure to deal with, but there are others that will keep aquarists awake late at night. Using an RO Di filter may not be a guarantee that all problems will be solved, but it certainly rules out many of them, and for this reason I strongly encourage its use in marine aquariums.

Bartek Stańczyk


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