ATU – which one?

A short video by Tony Brazao from

article by Gordon Lowe

This now common addition to the marine aquarium has been about for some time, I do not know when the first commercially produced unit came on the market (possibly the Tunze osmolator first released in 1985 and tunze claimed to have invented it) but I had a home made one running on my own marine tank back in in the late 1970s.

I believe the demand for the ATU (as I shall now refer to it) began with the need to maintain a stable salinity as salt water tanks became more popular and within the reach or the working man, it was only for people who did not drink or smoke as costs where high (that is, even higher than today in comparison) and more recently the introduction of live corals and the need for an even more stable environment. Just one more point to add to this era – we did not use RO water to top up, it came out of the tap, I know how many are cringing at this point but it’s not that bad when you consider we used to bleach our dead corals in domestos before adding to the aquarium to get that natural “white” look!

WHY DO WE NEED ONE?……Simple answer – we don’t, but unless you are present 24/7 to manually do the top up your salinity is going to vary, how much depends on how often you can check it – the salinity in the open ocean does not vary, all that rain has little effect on the vastness of oceans

WHAT THE ATU DOES and HOW…….Whatever the brand the ATU operates on the same principal, using a water level sensor (there are a few types) the sensor will detect a *drop in the water level, send a signal to a pump inserted in a reservoir of RO water and tells the pump to operate thus returning the water back to its intended level thus ensuring salinity is stable

*water level drops as a direct result of evaporation. This results in all water that is held in an open container whatever its temperature, what is important is to be aware that what evaporates is pure water and salts etc do NOT evaporate, this is the reason the water held in the reservoir for topping up needs to be preferably RO water


Basically down to 3 starting with simple probe detection where 2 probes are inserted in the water, water is a good conductor of electricity hence this information can switch a pump on or off depending on the presence of water – unreliable method to say the least so will not dwell on this

Next comes the horizontal or vertical float switch, well established it most commonly uses a reed switch internally which is activated by a magnet contained within the surrounding float, as the float moves up and down it activates the reed switch which in turn sends a signal to the pump
It has its good and bad features when used in the aquarium hobby, its reasonably priced and a well proven method of water level detection, unfortunately in our hobby we usually have “critters” intentionally or otherwise they crop up and can jam the float in an unwanted position, hence we can end up with the ATU either not operating or worse still operating when not required and this is where we end up with the ATU dumping the entire contents of the reservoir into the aquarium, depending on how much water is in the reservoir in relation to the tank volume this can be disastrous – the other way this type of sensor can fail is for the contacts inside the reed switch to weld together and making a circuit when not required, this is however unlikely if the design of the switching circuitry has allowed for excessive current that would result in this scenario –ie poor design, I will come to that in a bit!

Third and last type of sensor in common use more recently is the infrared sensor, arguably the best method as there are no moving parts to get physically jammed, basically sealed into the sensor is an infrared transmitter and its receiver, detection of water reduces the amount of light between the two and subsequent control circuitry will detect this and act accordingly


Mounting of the sensor is as important as the sensor itself, if it is not secure then it might as well not be there, suction cups on their own are inadequate and at some point will fail making the ATU inoperative, ensure its mechanically or magnetically secure, if you cannot do that then silicone in position, it should not however be necessary if you choose carefully.


That deals with the sensor, let us move onto the control circuitry, if there is one! I say that because there are some ATU’s on the market that do not or its very basic, simple control is not a problem so long as it does what is required – that being to ensure safety measures are in place should any component fail (and they can no matter how much you invest)

The price of the ATU very often reflects what safety measures are included, although not always the case you will be paying for an established brand name so research feedback from others that have purchased and the best way to do this is via the internet with particular note of postings on the many reef forums.

Onto the reservoir for the RO water, for some reason very few manufacturers supply the reservoir, it could be because there are so many different customer requirements and ultimately depend on the space available to you – ensure the container you use is food safe, there are many plastics that can leach contaminants into the water, a general guide is avoid coloured plastics unless you can ascertain its “potable”.

Finally the outlet – think it’s not important? It is, very much so, position it incorrectly and you potentially have water where you do not want it!

Make sure it is secure, or positioned so it is not going to move – ensure it is the correct height, sounds simple yet many make the common mistake of positioning so that back syphonage can occur and result in water either overflowing or salt water being returned to the reservoir.


REMEMBER – A GOOD ATU WILL ENSURE A STABLE AQUARIUM (and a cheap will let you down sooner or later)








couple of updates to the sensors, now available is the capacitive sensor which detects the water level a completely different method (capacitance suprisingly) to simplify its a similar method to detecting your finger on the touch screen of your mobile phone – this detects the water level through the glass or acrylic but it does not actually have to “see” it so it can be coloured or opaque, its one drawback are limitations to glass thickness which degrades sensitivity over 9-10mm

then there is also the new type optical sensor used by reefloat which hopefully does away with issue resulting over time with the conical type optical sensor where a droplet of water on the point would cause false detection

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