There is a built in 10 minute timer that once the sensor trips and activates the pump that requires the pump to move enough water from below thr min sensor to above the max sensor otherwise the unit will cut power to the pump and sound an alarm.

This effectively means that you need to measure the dimensions of your tank to determine how much water your pump needs to move in order to stay in this time limit, in my case I needed 7 gph for my 20 gallon tank (2 TOM aqua lifters).

What's this mean? Well The biggest thing is that it means that you most certainly do not want to use a TOM aqua lifter and I really don't understand why it is a paired item with this device.

It also means that if you have a sump setup your pump MUST be strong enough to overcome the water lost in the outflow.

For an example, I bought this item with a goal of using it to return water from my DIY aquaponic grow beds back to thr tank. It's a horizontal set up so water simply can't just fall back into the tank. I tried to keep the outflow constant but that meant I effectively reduced the total gph of the returning water below the nessessary threshold. In order to meet the threshold, I would either have to cut power to thr outflow pump or valve it down so that less than 1 gph was reaching the grow beds.

I can appreciate the idea of a failsafe alarm to let you know that there something wrong with the pump, but what I can't understand is why it cut power to the pump pretty much assuring that even if the pump was running at reduced capacity but still otherwise functional that it would not accomplish its job.

Now here's the catch 22, a number of reviewer's have commented about needing to make sure that the pump isn't so strong that it pushes so much water back into the tank that it goes up to the alarm sensor so there's that too!

In short this sensors so called 10 minute failsafe timer forces you into a Goldilocks paradox where you need a pump that is strong enough to get enough water back into the tank within 10 minutes, but not so strong that it overflows and pushes it pass the alarm sensor.

So what does this all mean? Will simply put you have two options:
1. Do the math and determine how much gph you need and buy a water pump that is 10-20% ( i'm just pulling a percentage out of my ass but it seems like a safe amount) above that amount ( don't forget to subtract if you have a sump!)
2. Buy a sensor that doesn't have a stupid killswitch timer built in.

So I have a 60 gal. cube saltwater aquarium with a 10 gal. sump, a 3 gal reservoir, a 3.5 gal. ATO Tom Aqua lift pump. The first unit I received was missing the magnetic attachment, but I was able to get one from Hydor customer service within a week. The unit worked great for about a month but failed while on a 4-day vacation! Luckily the house sitter was able to fill up the sump daily. I emailed Hydor and followed their instruction below:

"Please check for any forward siphoning and the pump before trying these steps below:
1) Turn off the ATO system
2) Set the sensor so that "Max" is at the water
3) Turn on the ATO system
4) Wait 1-1.5min for the ATO to go through the cycle (flashing)
Once the lights stop flashing, are the "Min" and "Max" LEDs on?
If yes, go to step 5
If no, go to step 6
5) Pull the sensor so that the "Min" is above water
After a few seconds the "Min" and "Max" LEDs should be off and the pump should be filling. If the water went to "Alarm" then the pump is a bit too powerful, turning down the pump will fix this issue.
6) Unplug the pump and plug it directly into a wall outlet, does the pump push any water?
If yes, you may need to send it back to us for inspection/replacement
If no, then you have a defective pump.
You may also test the unit with a low voltage LED lights instead of the pump to see if the ATO is working properly. (Max input to the ATO plug is 50W)
Please also keep in mind that the filling time is 10 min, a small pump may be powerful enough to push the water when the reservoir water level is high, but once the water is low the pump may not be strong enough to push that extra distance of head pressure."

Well the unit was defective so I sent it back and got a replacement with 2 weeks. The second unit worked great for about 4 months and now I'm back with the same issue of constant beeping and not filling up to the proper level! It seems the unit works fine until the first prolonged alarm event, after that the unit is never the same again. In retrospect the low cost ($58.99) Amazon has not been worth the time, effort, and the constant middle of the night alarms of annoying beeping!!

I bought this to use on my JBJ 8 gallon nano tank - using with a TOM Aqua-lifter pump. The minimum and maximum levels are too far apart for a very small tank. Even though I'm using the Aqua-lifter pump as directed (not more than 30"), the pump cannot seem to lift enough water in 10 minutes to bring the level to the maximum (middle) prong. On an 8 gallon tank - that's only 1/2 gallon at the max. Then, when I re-boot the Hydor controller - it's SUPPOSED to keep filling - but it does not. I even got a SECOND controller and it does the same thing. Not a big deal IF you never go out of town. But 1/2 gallon in an 8 gallon reef tank can change parameters enough to kill fish and corals. Another reviewer on here said to use the TOM AQUA-lifter with this. I'm going to have to buy a stronger lifter pump - but that doesn't change the fact that 1/2 gallon of evaporated water in a small system is good. The bottom 2 prongs NEED to be MUCH closer together to keep parameters in line.

I was tired of adding fresh RO water in my Sump almost daily. My 75 gallon evaporated fast. I researched ATO systems on youtube and this setup was one suggested. What I did:
I have a 5 gallon bucket of RO water beside my aquarium. I drilled a hole near the very top rim to place airline tubing in, and used the included tubing holder (via sticker) to the tube to keep it from floating around. It pulls water from the bottom. I use the suggested water lift pump (Tom) to pump it into my sump. I plugged it into the electric outlet that is connected to this hydor controller. Also out of the controller there is a sensor. I installed it very easily via the glass magnet inside my sump at my desired water level. It has 3 sensors, an overflow if it goes too high (and it alarms), and a middle one that communicates when to turn the power on to the toms top off pump, and a low level sensor which also alarms. The alarms are a great plus! If my return pump was to quite working and the sump begins to fill up it will alarm, and if I lose overflow suction and the sump tries to pump dry it alarms. It will catch either of these with the alarm way before they become a problem. But the main reason I have it is it is a great inexpensive way to have an ATO. Now my protein skimmer works phenomenally because of an exact constant sump water level, my salinity doesn't do two day cycles of high and low due to the lack then addition of RO water, and lastly, no more bending down and pumping water in my sump from 5 bucket every two days! I can just add water to that RO water bucket once a week! I can go on small vacations finally along with using an auto feeder for my FWLR tank!

Does exactly what it is supposed to. I wish it had a few additional features. 1. It needs to be adjustable on the auto shut off (alarm) fill time up or down. I can see this thing on a large main display tank defaulting and I just got a large tank I will be putting into from my 55 gal. 2. They need to include a suction cup that fits the mount for the unit. It is currently set up to use a screw to attach and I like this unit to be on the side of my tank so I can see it from my desk and quickly get a glimps of what it is doing but refuse to place a screw into the outside of my stand. 3. There also need to be an alarm acknoledgment button. A button that simply lets the unit know you are aware of the alarm that will silence the audiable alarm for only that alarm that time that allowes the unit to be quiet while you deal with that alarm and then reactivates when a new alarm happens. This would keep folks from shutting the unit off to shut it up and forgetting to turn it back on after dealing with the issue.