Many have come up with various numerological interpretations of the number 153 in John. I believe this to be reading into the text things not intended by the author. And, as the two previous answers to this questions illustrate well, this numerological method allows for several different interpretations of the same passage. Each of the words in the Bible, both Greek and Hebrew, adds up to a number. It is no wonder that of these over 10000 words (most with several different tenses/forms), one can use arithmetic to derive one word/number from other words/numbers as one sees fit, and read meanings into it. To illustrate my point, here are some other numerological interpretations of 153:
-Jerome claimed there were 153 species of fish, thus the catch became a symbol of a fruitful mission as fishers of men.
-Emerton notes that the streams of living water flowing from the temple in Ez 47:9-10, will have fishermen standing along the shore, from En Gedi to En Eglaim. "Gedi" has the numerical value 17, and "Eglaim" has the numerical value 153, and 153 = 1+2+3+4+.....+17. Thus the number represents all the fishermen.
-Augustine also noted that 153 = 1+2+3+4+.....+17. 17=10+7, which is the ten commandments + the seven spirits of God.
-Gregory the Great reaches 17 the same way, but multiplies it by 3, the number of the trinity, to get 51, and by 3 again, unto perfection, to get 153.
-Others break 7 further down into 4+3, the number of walls in the new Jerusalem + the trinity
-Yet others observe that 17 is the number of loaves of bread in the feeding of the 5000 + the number of baskets picked up after (=12+5).
-Others note that 153 is the numerical value of the phrase "The church of love" or "the children of God" or "Cana G" (+ "Cana in Galilee").
-Some see the number made up of "Simeon, Bar, Jonah, Kephas".
-Thoma finds 153 to be a reference to ICTHYS (a common early Christian acronym, standing for "Jesus Christ, Son of God, Savior").
As D. A. Carson observes, "Large quantities of ink have gone into explaining why there should be 153 fish. At the purely historical level, it is unsurprising that someone counted them, either as part of dividing them up amongst the fishermen in preparation for sale, or because one of the men was so dumbfounded by the size of the catch that he said something like this: ‘Can you believe it? I wonder how many there are?'" I agree: this event made such a lasting impression on John, that he remembered the exact number of fish they picked up. Just as he remembered the name of Melchus, whose ear Peter severed. I read no more into 153 fish, than I do into the 2000 donkeys in 1 Chr 5:21.
Having said this, I'll try to answer the actual question:
It is clear that the fish that were "already on" the fire were not part of the 153 the disciples brought. Therefore Jesus had provided the fish himself, whether miraculously or by purchasing them. At this point it doesn't make much difference, for both the reader and the disciples know that Jesus is God and has all power: one miracle more or less won't change anything.
John would not mention this incident unless he were trying to tell us something by it. The main thing this story is communicating, as evidenced by v14, is that Jesus was physically alive, and the disciples knew it. A sub-point of this passage may be that just as Jesus was with the disciples before, he still was with them. Moreover, He still provided for them. Even though they were out doing their own thing, trying to provide for themselves, Jesus had already prepared what was needed - though they also brought their own fish to the fire.