Aquarium Water Is Just Water Right – Wrong!

When keeping an aquarium – be this coldwater, freshwater, marine etc then there is one thing you are definitely going to need other than the aquarium – water!

Sound obvious doesn’t it?

Well let’s face it if we don’t put water in the aquarium nothing is going to survive is it – it’s not as if you can fill it with custard!

But water is just water – isn’t it?

The answer to this is both yes and no. It is correct that water is just water in the sense that it is wet however the substances which is in the water is what makes the water we want to use in our marine aquariums different.

In home water various additional substances are added to it in order to make it safe for human consumption and also various other aspects which get into the water by, for example, agricultural practice (phosphates, pesticides, nitrate etc).

It is for this very reason that the majority of aquarists use reverse osmosis water.

There are various other techniques which people also use – natural sea water, deionised water etc, however normally reverse osmosis is the water of choice for the majority of saltwater aquarists.

So what is reverse osmosis?

Basically a reverse osmosis unit is a device which is attached to the home water supply and this water is then pushed through a semi permeable membrane with pores so small that only water will pass through. It then also passes through other chambers where the majority of other substances are removed.

Generally the reverse osmosis process is 95% or higher in efficiency. The end result of the reverse osmosis process if water which is safe for aquarists to use in their saltwater aquariums.

There are a couple of things which need to be noted about a reverse osmosis unit however.

The first is the waste that a reverse osmosis system creates. For every five gallons of water pushed through a reverse osmosis unit, normally 4 gallons or so will go to waste. There are various factors which reduce the wastage in a reverse osmosis unit, the main one being the amount of calcium in your tap water. It has been noted that aquarists with soft tap water get a higher volume of usable water to waste ratio than those with hard water.

The second is chlorine. Chlorine is harmful to the membrane which is at the heart of the reverse osmosis unit. For this reason activated carbon chambers are provided for the tap water to pass through before the membrane is reached. These are normally called pre-filters or micro-filters. This pre-filter does require changing based upon the manufacturers recommendations. I recommend that you adhere to these recommendations as replacing the membrane will cost more. In a lot of instances, dependent upon the make and model of reverse osmosis unit purchased it is sometimes more cost effective to purchase a completely new unit.

The third is that the reverse osmosis membrane must constantly remain wet. If the membrane is allowed to dry out then it will become brittle and not function correctly. If this is allowed to happen all you will effectively be doing in sieving the water not purifying it.

Reverse Osmosis devices can be purchased nowadays for very reasonable prices and are available in numerous sizes. Size is normally determined in how much water is processed per hour, therefore a 50gph unit will create 50 gallons of reverse osmosis water per hour when used in optimal conditions.

It is definitely worthwhile investing in a reverse osmosis unit if you have a larger aquarium as the initial water fill and subsequent water changes will require more water, however for aquarists who have smaller aquariums it may not be cost effective to purchase a unit. Fortunately a lot of pet shops are now selling reverse osmosis water so if you have a smaller aquarium then it might actually be cheaper to simply purchase you water from your local fish shop.

As with anything in this hobby the unit does need to be maintained. It does not require a lot of maintenance but the pre-filter on the device does need to be changed at regular intervals. This is normally determined by the manufacturer and I would recommend that you follow their recommendations.

Another thing that I normally do is occasionally test the water from the r/o unit. I test both for nitrate and phosphate to ensure that the unit is running at optimum efficiency.

Another method which aquarists use is to test for total dissolved solids. Meters can be used for this which you can use to test the water produced. You can also purchase inline TDS meters which attach to the reverse osmosis unit. Some reverse osmosis units have these devices fitted.


Aquarium Water Is Just Water Right – Wrong!
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  1. You really hit a home run here. The number one reason why a saltwater aquarium hobbyist gets out of the hobby is because they can’t maintain excellent water quality. An RODI unit is one of the most important pieces of equipment. It reduces the amount of phosphate and nitrate from entering the system, which reduces algae growth and enhances coral coloration. A good rodi unit should produce 0 TDS and a good ro unit should produce a (Total Dissolved Solids) of less than 5ppt. The RO membrance will have the greatest impact on TDS, and the carbon and pre-micro filters will have very little impact.

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