This sounds like quite an impressive and complicated product. I don't own an Aquaero controller and do not know all of the answers to your questions, but I will share what I think.

1 - Adding 2 x Aquaero controllers as Expansion devices - I think the answer to this is yes. Note the Expansion Aquaeros need to be set to Aquabus addresses 26 and 27 per page 49 of the manual.

2 - Aquabus X4 Expander - The manual says each Calitemp sensor MUST be plugged into a separate port on the X4 Expander, so 3 Calitemps will use up 3 aquabus ports. For all other devices with Aquabus ports, you can use splitter cables like #53214. It says that you can only use one X4 Expander, but it also says you can connect one aquabus device to a port that has 1 Calitemp sensor connected. If this is true for an Aquaero controller in expansion mode, you could plug an aquabus splitter cable into 2 of the X4 expander ports, then plug an Aquaero expansion controller and 1 Calitemp sensor into each of the aquabus splitter cables. The 3rd Calitemp sensor would plug into X4 Expander port 3. The manual does not list aquabus port numbers for Calitemp sensors. The Aquaero expanders have to be set to aquabus ports 26 and 27. They also have the Splitty9 (#53231) which is a 9-port fan splitter that can also be used as an aquabus splitter. This device provides 9 places to plug in aquabus devices, but I don't know if you can plug Calitemp sensors into it. I suspect the Calitemp aquabus ports are hard coded into the X4 Expander. If this is correct, then you could plug the 3 Calitemps into 3 ports on the X4 Expander, then plug the Splitty9 into the 4th port on the X4 Expander and plug the 2 Aquaero expansion controllers into 2 ports on the Splitty9.

The Max Number in the table on pages 41 and 42 of the manual is how many of that device type can be connected via aquabus. The numbers in parenthesis are the aquabus addresses that a device type can be set to. For example, you can connect up to 2 Quadro fan controllers and they must be assigned to aquabus addresses 28 and 29. For an Octo, it says the max number that can be connected is 1, but it does not list the aquabus number. Since there can only be 1, I guess it can only be whatever port number they assigned to the Octo, and they did not bother to list it in the manual. They also do not provide the aquabus addresses for the Calitemp sensors, which is why I think they are hard coded into the X4 Expander ports.

3 - Aquabus Low and Flow 2 - That's a good question. The aquabus table says you can bring in 2 x flow sensors from 2 x D5 Next pumps and from 2 x Aquastream Ultimate. The X4 Expander only connects to Aquabus High on the Aquaero so if Aquabus Low is now a flow sensor input, I think you could connect a flow sensor to Aquabus Low header of the main Aquaero controller. Whether the Aquabus Low (now flow) headers on the Aquaero expansion units will still function and be forwarded to the Master Aquaero when they are in expansion mode is unknown. Hopefully someone from Aquacomputer will chime in here.

4 - By Fan Amplifier Outputs - I don't know if the Fan Amplifier temps from the slave Aquaeros are forwarded. I think the output current is based on the Aquaero manual Section 13.4 (page 31) - Programmable Fuses. It says, "For fan outputs of expansion boards connected via aquabus (fan outputs 5-12) the programmable fuses will have a considerable delay before tripping because of the aquabus communication." This tells me that the fan port current info from both Aquaero expansion boards is monitored by the master Aquaero. It could not tell a slave Aquaero to pop a resettable output fuse if it was not monitoring the current flowing out of each fan port. Regarding heat, PWM fan ports do not generate a lot of waste heat. Ports driving DC fans will generate waste heat. How much depends on the fan speed. If the DC fan is running at 100% full speed, there will be very little heat. As the fan speed is reduced, the waste heat will increase.

If you only have 1 x DC fan, you may want to consider getting a Power Adjust 3 (#53166). The Ultra Version (#53167) comes with a firmware update that allows fan speed to be controlled by a temp sensor connected to the device. I don't know why Aquacomputer decided to make this firmware update into an "Ultra" version and charge $5 more for it. The Power Adjust 3 has a more efficient output drive circuit that produces less waste heat than the Power Adjust 2. I don't know if it is more efficient than the Aquaero output drive circuits. You can also get a nice heat sink for the Power Adjust 3 (#53181). The fan port on the Power Adjust 3 can output 30W, 36W with the heat sink. You can connect up to 8 of them via aquabus. Note that the port is DC only, no PWM.

Does the Aquaero make the Fan Amplifier temps available as a sensor source? An Octo controller does not, but it has all PWM outputs. I have a Power Adjust and Fan Amplifier Temp does show up in the Alarms section with fixed Alarm triggers at 80°C and 100°C. These alarm trigger temps are not user-adjustable. It can also be used as a data source. The Aquaero manual is not clear about this, but since it can drive DC fans, Fan Amplifier temps are a concern. I don't think you need to monitor the Fan Amplifier temps for outputs set to PWM. I would just monitor the 1 x DC Fan output. If you use a Power Adjust 3, it will probably report that temp to the master Aquaero. Again, hopefully someone from Aquacomputer will chime in.

Final Thoughts - Do you really want to control fan speeds based on the Fan Amplifier temps, especially since you only have 1 fan port driving a DC fan? Normally, fan speeds are controlled by coolant temp, or coolant temp minus ambient temp (delta-T). It seems odd to use the Fan Amplifier temps, which will be close to ambient for all the PWM ports. I could see monitoring the one DC port Fan Amplifier temp and using that temp in a virtual sensor as a hard limit, but not the primary data source for fan speed control. Make a virtual sensor based on coolant temp or coolant temp minus ambient temp, then add in the 1 x DC Fan Amplifier temp and set up the virtual sensor so if this temp exceeds a value, it overrides or scales back the "regular" coolant temp based speed curve. Also, since you only have 1 x DC fan, why not get an Octo instead of another Aquaero? Supporting 4 flow sensors is the only reason I can think of, and that depends on if the Aquabus Low header, now a Flow sensor input, is reported from a slave Aquaero to the Master Aquaero.

If the Flow Sensors are High Flow Nexts, High Flow USB, or MPS, you can connect all 4 of them directly to the Aquaero Master via Aquabus since the table says it supports up to 4 flow sensors. Their aquabus cables could plug into the Splitty9 that is connected to port 4 on the X4 Expander. Then you don't need to worry about the Aquabus Low/Flow Sensor inputs on the slave Aquaeros..You also did not mention what pump(s) you have. If D5 Next or Aquastream Ultimate, they each have a flow sensor header, as does the Octo. I connected the Signal output of my High Flow Next to the Flow header on my D5 Next, so the D5 Next will use that flow rate instead of the inaccurate virtual flow rate. If I connected the D5 Next to an Aquaero controller via Aquabus, that flow rate would be available to the Aquaero according to the table in the manual.

Hope this helps!

I made an Aquaero & Aquasuite Setup Guide over on OCN a number of years back that covers how to flash the Aquaeros into Slave units.

Here's a link to the guide https://www.overclock.net/threads/aquaer…-guide.1726492/

Here's is a quote from the guide on how to do it.

"If you are adding an Aquaero LT to have four more fan control channels, you will need to flash the Aquaero LT into a slave unit to work off of the main Aquaero XT. Nothing can be connected to the Aquaero LT unit except for the five pin USB cable.

Turn the computer off and turn the power supply off with the switch on the back of it, or unplug the power supply from the wall. This removes all power from the USB cables. Connect a USB cable to the motherboard or the Hubby7 if you are using one. Again be careful to connect the USB cable in the correct orientation.

Turn the power supply back on, start the computer, and then start the Aquasuite software. Now there will be another Aquaero Tab on the left pane of Aquasuite, perform the firmware update in the System Tab.

Next on the same Aquaero>System Tab there is another section at the bottom called "Expansion Devices", this is where you reflash the Aquaero LT into a slave unit. Press the "Reprogram Device" button and in about a minute it is flashed into a slave unit.

Another warning states the slave unit will not be recognized by USB any longer and will only work over the Aquabus connected to the main Aquaero XT. Also keep in mind that the only outputs that will work on the slave unit are the four fan channels, the temp sensor inputs, and you can add a second flow meter to the Aquabus low port. All other outputs are now disabled.

Power everything down again; you can disconnect the USB cable as it is unusable. Connect the Aquabus high port to a Hubby7 or the Aquabus high port on the Aquaero XT. Then you can connect your wiring for the extra fan channels to the Aquaero LT. You can actually add a third Aquaero for up to 12 channels of control if you need more channels.

Now in the Aquasuite software, click on the second Aquaero Tab in the left column it is going to show a USB symbol with a red X through it. The right pane shows "The device has no USB connection or is not recognized by the system". You can go ahead and click on the button to "Remove device from Aquasuite" as there is no use for that tab anymore."

Good luck with your project!