given that the aquabus cables are basically PWM fan extensions (unshielded), i suspect it is a very slow protocol that doesn't care much about which type of cable is used '
could be some kind of multipoint RS485, but i really don't know. if that's the case, it will work just fine on cat 5. worth testing if you already have the devices on hand

I agree it should work fine but I could not find any specific info on what protocol and signal transmission standard Aquabus uses. I suspect it is CAN buss or MOD buss running at 19.2kbaud over RS-485. Category cable is rated for much higher speeds so it should work. The Aquaero manual says, "In order to avoid aquabus data corruption by electrical interference, aquabus cable length should be kept as short as possible." which is not very helpful. I guess only Aquacomputer knows for sure. Maybe they will chime in here.