It would seem to me that Wittgenstein in the TLP, professed a highly nominalistic view of the world. If anyone objects to this then all the merrier the discussion; but, that is my interpretation of the Tractatus. However, in the Investigations he directly addresses the issue and says:

'We are not analyzing a phenomenon (e.g. thought) but a concept (e.g. that of thinking), and therefore the use of a word. So it may look as if what we were doing were Nominalism. Nominalists make the mistake of interpreting all words as names, and so of not really describing their use, but only, so to speak, giving a paper draft on such a description'. — (L. Wittgenstein Philosophical Investigations, I/§383)

Any previous passages or later passages don't help elucidate the mystery behind talking about phenomena and concepts. At face value, this would seem as a form of indirect realism or even anti-realism if you take a harder reading of the passage in isolation.

So, what is the difference here being made about 'phenomena' and 'concepts about phenomena'? And, on what grounds can one say that there even is a difference between the two? It would seem as though one were to ask a fish, 'How does the water feel like?'.