You can't monitor your the Wi-Fi network using tethering even if Android supports promiscuous mode for the Wi-Fi chipset.
Reason:
Tethering does NAT internally and assigns you an IP in a 192.168.* private range via a DHCP daemon running on your phone. There's no way you can see pure Wi-Fi traffic this way.
What you can do:
Have a custom firmware installed (e.g. CyanogenMod) and do a tcpdump on your phone itself (if it supports both PROMISC mode and tcpdump)
