It depends on the zoogeographical region, freshwater or brackish or salt. General keys are based on the most typical (in author's opinion) species. In first approximation such general keys can be used, but you should support tham by specialised keys, e.g. with regional or world fauna books. In post soviets countries f. ins.  when identifying Rotifers we use normally perfect Kutikova book, when have to do with Chidoridae – Smirnov Fauna of the world (1971) (in Russian). As well, Cyclopoidea by Monchenko, Harpacticoidea by Borutzky etc. So, you should define more accurately you interest.

Andrey