Is this ECU control box compatible with the aqua hot WPE-300-901 We sent the original part number to a parts house and we got the aqua hot part instead.

It is very possible that this Webasto control box is suitable for your Aquahot system. Initially please could you send a photo of your existing control box ECU to sales@butlertechnik.com to help us confirm which model and build year your Aquahot system is? The later Aqua hot systems have a black plastic ECU of similar size fitted and are based on a Webasto DBW 2010 SENSORIC heater. This control box is NOT a suitable replacement for a SENSORIC version heater