Aquarium ATO Sensor Mounts and Dosing Container Holders: Float Switches, Optical Sensors, and Bottle Sizes

Most auto top off sensors are not round, so most clamp style mounts do not fit them. Only one ATO sensor in the whole hobby publishes a body diameter, and on a mechanical float switch the widest part is usually the moving float, which you must never clamp.

This guide covers two jobs in the same reef cabinet. The first half is about holding an ATO water level sensor at one exact height. The second half is about holding your dosing containers. If you came here for dosing bottle holders, jump to the last three sections. The sizing rule there is just as blunt: most reef dosing containers are square, not round, and the volume printed on the label does not tell you the shape.

How does an aquarium auto top off work, and where does the sensor go?

An auto top off, or ATO, watches your water level and runs a small pump to add fresh water when the level drops. The sensor is the part that watches. It has to sit at one exact height, because that height is the water level you end up keeping.

Water leaves a tank by evaporation, and only the fresh water leaves. Salt stays behind. That is why a reef tank needs the level held steady, and it is why the sensor's position is the whole product.

On a sumped system the sensor almost always goes in the return chamber, the last chamber before the water goes back up to the display. That chamber is the only one whose level actually rises and falls with evaporation. Most keepers also keep the end of the refill tube above the water line, so water cannot run backward down the line when the pump is off.

Float switch or optical sensor: which one are you mounting?

There are two kinds, and the difference decides how you are allowed to hold them. A mechanical float switch has a floating collar or ball that slides up and down a stem. An optical sensor has no moving parts at all. It reads a light beam inside a solid tip, so nothing on it travels.

Look at your own sensor before you buy anything. If any part of it slides, it is mechanical. If the whole thing is one solid piece with a clear tip, it is optical.

There is a third kind too. Some sensors read the water with two small metal electrodes instead of light or a float. Those are solid state as well, but the electrode faces have to stay clear, so nothing may cover them.

• Optical, no moving parts: Neptune Systems OS-1, AutoAqua Smart ATO, XP Aqua Duetto 2, Red Sea ReefATO+, Tunze Osmolator sensors.
• Electrode based, no moving parts: Innovative Marine HydroFill and HydroFill Ti, which use titanium electrodes rather than light.
• Mechanical float, with a moving collar or ball: CoralVue HYDROS float switch, JBJ float sensors, and the whole class of threaded float switches sold with an M16 or M10 thread.

Why does sensor height matter more than sensor brand?

Because the sensor's height is your water level. Move the sensor up 5 mm and your water line moves up 5 mm. Nothing else on the device changes that.

This is what makes an ATO sensor different from a pH or ORP probe. A probe just has to be in the water, so it can sit anywhere in the water column. An ATO sensor has exactly one correct height, and it has to stay there. If you are looking for a pH probe clip, a controller box bracket, or a heater clip, our probe, controller, and heater mounts guide covers those. This page stays on water level sensors and dosing containers.

Because height is the whole point, every mount in this family has real travel built in. The slotted brackets give 30 mm or 40 mm of straight run, so the sensor slides up or down and then its own nut clamps it wherever you want. The clip style brackets use one continuous channel that holds 22 mm of the sensor body at any height inside the travel.

Which ATO sensors take a round mount, and which ones do not?

Only three interfaces in the hobby can be sized from a published number. The Neptune Systems OS-1 optical probe, whose body is 12.70 mm across. An M16 threaded float switch, which mounts through a stated 16.5 mm hole. And an M10 threaded float switch, which states its thread but no hole size. Everything else either is not round, or publishes nothing at all.

This surprises people, so it is worth naming plainly. AutoAqua's own spec sheet calls its part a square optical dual sensor. Innovative Marine's HydroFill sensor is a rectangular block with exposed titanium electrodes. XP Aqua's Duetto 2 packs two sensors into one small clip housing and is never described as a cylinder. Red Sea's ReefATO+ is a slim titanium probe on a magnetic corner bracket. Tunze publishes no dimension of any kind for its sensor, so nobody should assume it is round.

Watch out for one number in particular. Several makers publish a maximum mounting thickness, often 12.7 mm. That is how thick a pane of glass their own magnet or clip will grip. It describes your tank, not their sensor. Sizing a clamp from it would be guessing.

Why should you never clamp a float switch at its widest point?

Because on a mechanical float switch, the widest point is usually the float itself, and the float has to be free to move. A clamp closed around it pins the float in place. A pinned float stops reading the water.

This is not a small detail, and it is easy to walk into. One maker publishes a 3/4 inch diameter for its float switch, which works out to 19.05 mm. The page never says whether that names the fixed stem or the travelling collar. On a vertical float switch, the widest measurement is normally the collar. Anyone who bored a clamp to 19.05 mm would have a good chance of gripping the moving part. That is why we do not sell a clamp for it, and will not until somebody puts calipers on a real one.

The honest rule is simple. On any mechanical float switch, grip a fixed feature only. That means the threaded boss, its own cap nut, the cable gland, or a magnet mount. Never the stem the float travels on, and never the float.

Our own generator enforces that rather than trusting anyone to remember it. Ask it for a clamp on a mechanical float and it refuses to build the part. It also keeps printed structure out of the float's path: one sourced float ball measures 28 mm across, so the panel brackets hold a 14 mm clear run beside the slot on every float switch part.

One thing this bracket is not. It is a bracket. It holds a sensor at the height you set it. It does not make an auto top off reliable, and we make no claim that it does.

Rim clip, wall bracket, or threaded panel: which ATO mount fits your sump?

Pick by what you have to attach to. A rim clip needs an edge to hang over. A wall bracket needs a flat face you can put two screws into. A threaded panel bracket is for a float switch that was designed to go through a drilled hole you do not want to drill.

The last one is worth explaining, because it solves a real problem. A threaded float switch is built to pass through a tank wall and clamp with its own nut. Most keepers do not want to drill their sump. So the bracket hangs on the wall and becomes the panel instead. The switch threads through the bracket, its own cap nut clamps it, and the float hangs on the wet side untouched. That is the strongest fit in this whole family, because the mating size is a published manufacturer figure rather than a guessed body diameter.

Two practical notes. The rim channel opens to 12 mm on the standard version and 18 mm on the thick rim version, and a thumb screw snugs it down onto anything thinner. That is a range, not a point fit. And nothing comes with hardware: each clip needs one M5 nylon thumb screw, and the wall bracket needs two screws you pick for your own wall.

For the general rim clip pattern and how to measure a tank or sump wall, see our probe, controller, and heater mounts guide. We do not repeat that walkthrough here.

• Rim clip: hangs over a tank or sump wall. Channel opens to 12 mm or 18 mm. Good for a return chamber with an open top edge.
• Wall bracket: screws to a flat face like a sump baffle, an overflow box side, or a cabinet wall. Two full length rails hold the panel 12 mm off the surface so the cable and any nut clear it.
• Threaded panel bracket: for an M16 or M10 threaded float switch. The bracket becomes the panel, and the switch's own cap nut does the clamping. Slot travel is 30 mm, or you can pick a fixed hole version with no travel at all.

Should you run two sensors instead of one?

Plenty of reef keepers do, and the reason is easy to follow: one sensor is one point of failure. A common setup is a main sensor at the working level plus a second sensor mounted a little higher as a backup.

We are not going to tell you that two sensors prevent anything, because that is not a claim we have earned or tested. What we can say is what we build. Two brackets side by side let you set two different heights, and the slotted versions let you fine tune the gap between them without unbolting anything.

If you are running two, mount them on separate brackets rather than trying to crowd both into one. Separate brackets mean you can move one without disturbing the other, which is the whole point of a backup.

Same cabinet, different problem: what does a dosing container holder do?

A dosing container holder keeps your dosing bottles upright and parked in one place, either on the cabinet floor or screwed to a cabinet wall. It is the same shape of problem as an ATO mount, which is why both live on this page: hold a piece of reef gear at a fixed position, in the same cabinet, under the same tank.

The part itself is a tray of bays. Each bay is a shell that the container drops into. The floor is open bars with gaps between them, and the upper wall is cut into fingers rather than left solid, so the graduations and the dosing instructions on your bottle stay readable.

Say the honest part out loud. Those open floor bars mean this is not a spill tray. It does not catch a leak, it does not keep your cabinet dry, and it does not protect anything from a chemical that gets out of a bottle. It holds a container upright and in one spot. That is the job.

How big are reef dosing containers, and how do you measure yours?

Check the shape first, then measure across the widest part of the body. Most purpose made reef dosing containers are square or rectangular, not round, so shape is the question that decides everything else.

Here is the finding that makes guessing dangerous. One packaging supplier sells a 1 gallon round bottle and a 1 gallon square bottle at the same volume, with the same neck size. Same litres, same cap, completely different holder. Volume does not predict shape and neither does the brand.

The supplement bottles most keepers already own are the hardest case of all. Tropic Marin, Red Sea and Aquaforest publish a volume and nothing else, and no retailer selling them publishes a dimension either. We searched all three brands across their own sites and six retailers. So we will not tell you a holder fits a named supplement bottle. We name diameters, and you check yours against the number.

1. Look straight down at the bottle. Is the body round or four sided? Do not go by the volume on the label.
2. Measure across the widest part of the body, with the label still on. Labels add a little, and nobody publishes how much.
3. Measure the height without the cap.
4. Compare your number to the bore. Our round bores run 2.0 mm to 5.0 mm wider than the bottle, wider on the big ones.

One bottle or four: how should a dosing holder mount and hold them?

Pick the number of bays to match your dosing schedule, then pick freestanding or wall mount. A two part schedule wants two bays. A three part reef schedule of alkalinity, calcium, and magnesium wants three.

Bay count runs into a printer plate at some point, so bigger trays ship as separate modules that lock together with a dovetail. At 91 mm, a two bay tray is still one piece at 198.5 mm wide, and three and four bay trays arrive as separate single bay modules. At 110 mm even the two bay arrives as two modules. That turned out to be the better product anyway, because the tray grows as your dosing schedule grows.

The cradle wall comes up about 30 percent of the bottle's height, with a floor of 35 mm and a ceiling of 110 mm. That puts the grip band on the lower third, where the weight sits when the bottle is full. Going much taller starts covering the graduations you read every week.

Tall skinny bottles get one extra feature. Any container taller than 2.5 times its narrowest footprint gets a strap band, with 20 by 4 mm slots at about three quarters of the bottle's height. A strap or a heavy cable tie runs out one slot, around the bottle, and back in the other. The strap is not included, and we do not claim it stops a tip. The band is there so you have somewhere to put one.

On the wall mount versions, the screw hole is a 4.5 mm clearance hole for a common number 8 wood screw. Screws are not included, and we publish no weight rating and no advice on what panel it is safe on, because we have not tested that. The dosing line pass through is a 6 mm hole, and that is a hole size, not a fit claim for any named tubing.

Frequently asked questions