Understanding Reverse Osmosis for Aquariums: How It Works, Pros, and Cons

In the aquarium hobby there are many instances where reverse osmosis water is used but to most people it can be quite baffling, if they have never actually had to invest in a unit. Hopefully this article will explain its uses and also exactly what it is.

It is mainly used for water softening which is ideal for discus, angelfish and in fact nearly all of the South American species, also it is now used with almost every marine or reef tank.

An RO unit (reverse osmosis unit) is used as a water purifier and in fact it will reduce the pH of the water as it pulls out 99% of all total dissolves solids, leaving the closest thing to pure water that you can get. This is particularly important where water quality has to be at its highest for the more delicate species of fish and where internal or external filter cannot clean the water to reach desired levels.

The water is so pure that it has to be re-mineralized before use, as it can become unstable due to the KH potentially dropping, which in turn can cause a pH crash.

How Does a Reverse Osmosis Unit Work in an Aquarium Setup?

So how does the unit work? Well basically osmosis is where two solutions of a different concentration will try to balance themselves out if a semi-permeable membrane is dividing them. A reverse osmosis unit also uses a semi-permeable membrane that is sited inside its filter housing. Water is introduced under pressure into the unit and as it hits the membrane, only the water molecules can pass through, the remaining undesirable trace elements and minerals are then drained off through a waste pipe. These units will also remove a lot of the oxygen present in the water so before use it must be aerated to get some back in.

There are many types of reverse osmosis systems, but for ease of reading I will explain the basic one - a three stage.

The membrane used in these units is of a delicate nature so pre filters are always included in the set ups. As soon as the mains water hits the unit, it will enter the first housing chamber, which will contain a sediment filter. As the name suggests this will remove the larger particles from the water and normally they are capable of removing anything as small as 5 microns in size. They should also be of a high enough quality to remove bacteria, spores etc. Normally these are replaced on average every four months, but the golden rule is to change it when it starts to discolor, normally turns brown over a period of time. This will be the cheapest filter in the unit to replace, prices average between 2 to 3 pounds for each one.

The water will then pass onto the next filter housing chamber; this will contain the carbon pre filter. This works along the same principles as using carbon in your tank, it will remove chlorines, toxins and any other chemicals that shouldn’t be there.

It is a useful tip to check with your local water authority as to which chemicals are used to clean the main water pipes as sometimes chloramines are used, this is a mixture of chlorine and ammonia, so make sure that the carbon pre filter is capable of dealing with this to prevent it getting through. Usually these are rated on the gallon age of water that they can treat before they need changing, usually six months of use is the max.

There are different types of carbon pre filter – CCB filters are coconut fiber carbon blocks, CBR’s that are slightly higher quality and normally deal with a higher gallon age of water, and granulated carbon filters. You can also purchase acid rinsed filters, specially designed for discus set ups to keep the pH level down.

The water has now passed through the two pre filters, and at this point is ready to enter the filter housing containing the membrane. There are two main types of membrane used, but the most common in household RO units are the TFC membranes. This stands for Thin Film Composite membrane; these are designed for mains water but will get damaged if in contact with chlorine, hence the need for the pre filters.

The other main one is the CTA membrane (cellulose triacetate); these are more for industrial units and are quite expensive to buy.

The filtered water will now leave the unit to be collected in the vessels ready for use, and the water that wasn’t allowed to pass through the membrane will leave the unit by a drain tube. On this tube will be a flow restrictor valve, this is what builds up the pressure in the unit to make it so efficient.

I have just described to you the workings of a basic 3 stage unit but they can be supplied in 4, 5, and 6 stages for extra efficiency. One of the main extras that can be added on is a DI unit which basically gives the water a final polish, and this contains a resin that is re-usable when rinsed.

Essential Maintenance Tips for Reverse Osmosis Units

There are many things to remember when using these units; always use clean barrels, buckets to run the purified water into, never use metal or dirty ones as this will just foul the water so you would be undoing what the filter has done.

The unit is dependant on a high enough water pressure from the mains to make it run efficiently, if the pressure is too low an additional pump can be fitted inline to increase it.

They are also dependant on the temperature; if it drops below 1 deg then the membrane will become damaged. A lot of people have their units fitted outside so if there is any danger of frost the unit must be brought indoors.

Never let the membrane dry out as this will render the membrane useless, membranes cost anything up to 30 – 35 pounds, so it is well worth looking after them, and normally they will last for 2 – 3 years.

The efficiency of your reverse osmosis unit can be tested with a TDS (total dissolved solids) meter, my mains water runs at 256 TDS, and after it has been through the unit the reading is 0 – 3 TDS. The meters can be added inline or handheld ones are available.

Aquarium reverse osmosis systems are a good investment if running marines or soft water loving fish so I do hope this article has explained the workings of them to you.