i just tested when the filter is turned off if it will have a siphon and thats no. so if you loose power only the water in the HOB
filter only goes into the sump.
i tested by sticking a siphon hose in my HOB
filter unplugged it then made a siphon from the HOB
filter to my sump. and soo as the HOB
filter was empty the syphon stopped not sucking any extra water from the tank.