Coralline growth on Tunze ATO sensor causing tank flooding

[coloradoreefer94]

Welp, its finally happened. My worst nightmare, I was woken up at 2 AM (thankfully from my watchdog alarm) to my 20g tank filled to the brim and overflowing. I immediately unplugged my ATO and removed the excess water, luckily salinity only dropped from 1.025 to 1.023. I diagnosed the issue and my Tunze osmolator nano 3 ATO sensor had coralline growing literally over the optical. Lesson learned. I need to manually clean this thing often, it had been about 9 months or so since the last time I removed the cap to actually clean it. Anyone else have this same thing happen to them? Ways to avoid ATO overflow issues like this in the future other than obvious cleaning of the sensor periodically?

 

[BryanM]

Hows coraline growing in a box with rodi water in it? That seems weird.

 

[coloradoreefer94]

Hows coraline growing in a box with rodi water in it? That seems weird.

The sensor for the ATO is in the return chamber of the tank.

 

[JGT]

Good husbandry is the answer. 9 months ain’t bad.

 

[MasterClassReefs]

Id start cleaning it every 6 months to avoid this issue. Im going as far in my system as to make the plumbing removable for cleaning and inspection on a schedule. Its work, but so is cleaning up a disaster.

 

[GoReefin]

Odd isn't there a mechanical float valve to stop this from happening on ATOs nowadays?

 

[MasterClassReefs]

Mechanical floats can get stuck too.

 

[BryanM]

The sensor for the ATO is in the return chamber of the tank.

classic brain fart. Clearly I needed more coffee yesterday.

 

[rtparty]

You need to lightly scrub it down every month or so or this happens. Coralline will grow where there is light

 

[56longroof]

Bi weekly maintenance.

 

[X-37B]

I run 5 and just cleaned the one in my 150. Salt buildup no coralline.
I use a cover made from foam pvc on my 15g aio. No light no coralline on the sensor.

 

[higher99]

I run 5 and just cleaned the one in my 150. Salt buildup no coralline.
I use a cover made from foam pvc on my 15g aio. No light no coralline on the sensor.

Hi, can you make a thread about how you keep your SPS corals in the nano?

I’m thinking about adding a few frags but I’m not dosing anything as of now. This is my IM Lagoon 25.

 

[X-37B]

Current 15
Thread 'IM 15 Nano Cube' https://www.reef2reef.com/threads/im-15-nano-cube.1099086/
Old 20

Thread '18" AIO Nano Cube Stag System' https://www.reef2reef.com/threads/18-aio-nano-cube-stag-system.883090/

 

[NanoSteam]

Also an AIO user with a Osmolator. Cheapest solution was filter sponge to block light. No algae growth.

 

[X-37B]

Also an AIO user with a Osmolator. Cheapest solution was filter sponge to block light. No algae growth.

Great idea!

 

[NanoSteam]

Great idea!

Credit to @TheNative192

 

[rvitko Verified Sponsor]

This should have locked it off, no top off, I would dig deeper on this, this system is designed to fail off and the algae would trick it into thinking it is in water. You may have a siphon issue.

 

[55semireef]

This should have locked it off, no top off, I would dig deeper on this, this system is designed to fail off and the algae would trick it into thinking it is in water. You may have a siphon issue.

You might be conflating the Osmolator 3 with the Nano (what the OP has). The Nano tends to have more flood issues because it relies on just a single optical sensor with no independent backup to catch a failure. If that one sensor gets blocked by coraline algae, algae film, bubbles, or even slight misalignment, it can misread the water level and keep the pump running longer than it should. On top of that, it is a simpler system with fewer built in safeguards, so there is less protection if something goes wrong. The full Osmolator 3 has a second safety layer and smarter shutoffs, so even if the primary sensor has an issue, there is another mechanism to stop the pump before it turns into a flood.

 

[rvitko Verified Sponsor]

In both cases the optic sensor is the same, they do not run if the sensor is covered and light cannot pass through, the algae has the same effect as being wet. The 3151 only lacks the back up sensor and replaces it with a very short time limit, so a fill cannot exceed 3 liters.

 

[55semireef]

In both cases the optic sensor is the same, they do not run if the sensor is covered and light cannot pass through, the algae has the same effect as being wet. The 3151 only lacks the back up sensor and replaces it with a very short time limit, so a fill cannot exceed 3 liters.

Yes, both systems use a similar optical sensor, and yes, the nano version relies on a time based cutoff instead of a physical backup sensor. On paper that sounds like a reasonable safety tradeoff but in practice, the difference is how failures behave, not just how they are prevented. With the nano, if the sensor misreads due to bubbles, film, or placement, the system has no way to independently verify the water level. It just runs until it hits that time limit. So while it may not run forever, it can still add a meaningful amount of extra water each time it triggers incorrectly. Over multiple cycles, that can slowly push salinity down or contribute to a small overflow depending on your setup. With the full Osmolator 3, the second sensor is not just a backup, it is a completely separate check on reality. If the primary sensor is wrong for any reason, the backup sensor shuts the system down immediately regardless of timing. That is a fundamentally different type of protection because it does not rely on assumptions about flow rate or volume.