Since no one answered so far, I looked it up myself and it turns out: although за usually requires prepositional case or accusative, in this context (что это за ...) it is in fact followed by nominative. Czech and Slovak require accusative for this construction, but in Polish it is the same as in Russian and they also have nominative there. (https://russian.stackexchange.com/questions/7804/nominative-after-%D1%87%D1%82%D0%BE-%D0%B7%D0%B0)
My answer of Tim, what kind of a fish is this? was rejected. Both should be correct but my answer is more likely in the USA. The meaning is slightly different. If you are holding a fish you just caught my question would be used 99% of the time. If there is a cooked fish on your plate, the given question is better.