i believe we can conclude the ID of this species as Brachysynodontis batensoda.
basically it is spending most of its time upside down under the driftwood which fits exactly to the description on scotcat (http://www.scotcat.com/factsheets/s_batensoda.htm):
B. batensoda spends most of its time inverted under branches or logs
basically it is spending most of its time upside down under the driftwood which fits exactly to the description on scotcat (http://www.scotcat.com/factsheets/s_batensoda.htm):
B. batensoda spends most of its time inverted under branches or logs