(1) Aristotle makes a distinction between PRIMA SUBSTANTIA and SECUNDA SUBSTANTIA. Think of the former as an individual substance (i.e. a dog, cat or lamb) and the latter as a reference to an essential classification of being (i.e. essences). However, as Joshua Hoffman and Gary S. Rosenkrantz point out in The Divine Attributes (Malden, MA: Blackwell Publishing, 2002), pages 23-24, it is probably more accurate to think of Aristotle's developed theory of SUBSTANTIA as a delineation of form and matter.
(2) Tertullian's use of SUBSTANTIA in Adversus Praxean is fraught with many conceptual difficulties. Harnack proposed juristic definitions for SUBSTANTIA and PERSONA that have pretty much been rejected by modern scholars. One of the best studies I consulted for my M.Th. thesis was George Stead's "Divine Substance in Tertullian," JTS N.S. 14 (1963): 44-66. Another writer who addresses Tertullian's use of SUBSTANTIA is Eric Osborn. See Tertullian: First Great Theologian of the West (New York: Cambridge University Press, 2003). In my thesis, I argued (based on Tertullian's De Anima 32) that the apologist utilizes SUBSTANTIA vis-a-vis the Godhead to mean "a concrete particular." That is to say, Tertullian is primarily conscripting the Stoic notion of SUBSTANTIA, as opposed to the Aristotelian concept of PRIMA or SECUNDA SUBSTANTIA. Stuart G. Hall writes that Tertullian uses "substance" (in the context of Adversus Praxean) to mean "a being." See his study Doctrine and Practice in the Early Church (Grand Rapids: Eerdmans, 1992).
My thesis later became the book Angelomorphic Christology and the Exegesis of Psalm 8:5 in Tertullian's Adversus Praxean. I hope that it contributes to an understanding of Tertullian's use of substantia, among other things.