Description

The Neptune Systems A3 Apex Jr answers a question that the hobby has asked for years: what is the minimum viable version of the Apex that still delivers genuine protection and peace of mind? The answer is this — temperature, pH, water level and leak detection, WiFi, Apex Fusion and two AquaBus ports ready to expand into full control the moment you want more.
At $299, the A3 Apex Jr is the lowest price Neptune Systems has ever put on an Apex device. It is not a compromise product — it is a deliberate, honest starting point for reefers who want the Apex ecosystem without paying for capabilities they don’t yet need, and who want the option to grow into those capabilities over time without starting over.
What the A3 Apex Jr monitors out of the box
Temperature is the parameter that kills tanks fastest. A heater that sticks open, a chiller that fails during summer, a power outage that allows gradual cooling — the Apex Jr monitors temperature continuously and sends an alert to your phone the moment it drifts outside your defined safe zone. The included probe is lab-grade and double-junction, holding calibration longer than standard probes and delivering stable, accurate readings over time.
pH monitoring is valuable for detecting dosing anomalies that would otherwise go unnoticed for days. A stuck dosing pump delivering too much alkalinity will shift pH measurably before corals show visible stress. A CO2 scrubber that needs replacing shows up in a pH trend before it becomes a crisis. The Apex Jr watches this number continuously and alerts you the moment it moves outside your set limits.
Water level monitoring via the included MOS Magnetic Optical Sensor can serve multiple purposes depending on where it’s placed: low-level warning in the sump to detect slow leaks or evaporation issues, high-level warning in an ATO reservoir that’s overflowing, or dosing container level monitoring to alert before a pump runs dry. The MOS is low-profile and mounts magnetically on material up to ¼” thick.
Leak detection via the included LD-2 surface leak sensor protects the floor beneath the aquarium stand. Water contact with the sensor triggers an immediate notification — the kind of alert that turns a potential flood into a minor inconvenience caught before it causes damage.
Apex Fusion — the same platform as every other Apex
The A3 Apex Jr connects to the same free Apex Fusion cloud platform used by the standard Apex EL and Apex A3. Everything accessible from the Apex Fusion app on the full controller is accessible from the Jr: real-time parameter monitoring, parameter logs, customisable alerts, reminder scheduling, dosing records and the dashboard layout you can configure exactly as you want it. The Jr connects via built-in WiFi — setup takes minutes through the Apex Fusion guided wizard.
The iOS app delivers push notifications directly to iPhone and iPad for temperature, pH and leak alerts, so you don’t need to depend on email or text message for time-sensitive warnings.
Two AquaBus ports — the path to a full controller
This is what separates the Apex Jr from a simple standalone monitor. The two AquaBus ports on the Jr accept any Neptune Systems Apex module or accessory — the same ones that work with the full Apex EL and A3.
Add an EB832 Energy Bar and the Jr becomes a complete controller with 8 individually monitored AC outlets and 1LINK ports for DOS and WAV pump integration. Add a PM2 module for conductivity/salinity monitoring. Add a PM3 for dissolved oxygen. Connect a Neptune Trident for automated alkalinity, calcium and magnesium testing. The Jr can be expanded to match the full Apex A3 feature set at any time — the only limitation is that it ships with WiFi-only internet connectivity, with no Ethernet port.
Who the A3 Apex Jr is for
It’s the right choice for three types of reefers. First, those who are new to aquarium controllers and want to start with monitoring before committing to full equipment control. Second, those who already have working equipment schedules on a timer and simply want the protection of parameter monitoring and alerts. Third, those who want to enter the Apex ecosystem at the lowest possible price and expand it methodically as their system grows.
It is also the right second controller — a dedicated monitoring unit for a second tank, a quarantine system or a grow-out tank where full Apex control isn’t justified but basic parameter protection is.
Specifications:

Monitoring: Temperature + pH + water level + leak detection (all included)
Expandable monitoring: Salinity, ORP, dissolved O2 (via Neptune modules)
Connectivity: Built-in WiFi (no Ethernet port)
AquaBus ports: 2
FMM ports: 2
Platform: Apex Fusion (free cloud, iOS app)
Equipment control: requires EB832 Energy Bar (sold separately)

In the box: A3 Apex Jr base unit, power supply, temperature probe, lab-grade double-junction pH probe, MOS Magnetic Optical Sensor, LD-2 Leak Detection Sensor, pH calibration solution packets (7.0 and 10.0)