In John 6:5 he tells them to fetch bread (no fish). In Mark 6:38, only loaves. 

Notice he only blesses the loaves, to multiply them. Not the fish. And the loaves are all that is gathered up at the end. One must ask, if this is the case, and Jesus only mentions loaves in his retelling, did he only multiply the loaves? Especially given the early church fathers who had the original texts in front of them (which we no longer have) recorded this miracle as only loaves. The quick little fishy sentence in John seems suspect.

“Do ye not yet understand, neither remember the five loaves of the five thousand, and how many baskets ye took up?” - Matthew 16:9

And John 10 again for reference:

“Jesus then took the loaves, gave thanks, and distributed to those who were seated as much as they wanted. [He did the same with the fish.]

12 When they had all had enough to eat, he said to his disciples, “Gather the pieces that are left over. Let nothing be wasted.” 13 So they gathered them and filled twelve baskets with the pieces of the five barley loaves left over by those who had eaten.”