Batrachomoeus trispinosus belongs to the Batrachoididae family (toadfishes) grows to about 12 inches (30 cm) so a standard sized 29 gallon tank is too small. A 40 gallon breeder tank (36"x18"x16") is a good minimum size for this species. Correct, they don't move around much as they sulk 90% of the time, however a 29 gallon tank is still too small. Batrachomoeus trispinosus is a brackish to marine species so aim for a higher end salinity (eg at least 1.010 specific gravity). These guys are predators so make sure fish housed with it are at least half it's size, they have really wide mouths. Also, avoid a gravel substrate.
Hope this helps.
