Is this one specific fish in Polish ? Like THE fish?Or could this also mean "fish" in general. Like I'm cutting fish.
It can either mean "one fish", or "fish meat" as a mass noun.
English plural "fish" is "ryby" in Nominative (and here, in Accusative, it also would be 'ryby').
If "dla czego" means "for what", why should "dlaczego" mean "why" instead? And if you take another slavic language like Russian, is has very same "для чего" (dlya chego) meaning exactly "for what". This makes little sense.
"For what are you cutting the fish?" or "What are you cutting the fish for?" and "Why are you cutting the fish?" are very close. The first two ask for the specific purpose, the second is slightly more general. However, they are both within the meaning of "why", and so equivavslent to dlatego. Note that some people don't like sentences ending in prepositions like "for", but it's not a strict rule to exclude them.