Actually.....I suppose you could classify Hoplias and Erythrinus as 'ancient' fish. There is a widely held school of thought that these fish may be the most primitive extant taxon of the Characins (tetras and their relatives) and may be very similar to the ancestor from which the rest of the Characins are derived.
The Coelacanth (Latimeria and others) are representatives of a very old lineage of fish but whether they pre-date the lungfish is a matter for some debate. Bony fish of any sort, however, are certainly not the oldest underwater creatures. The Agnathans (lampreys and hagfishes) certainly pre-date the Coelacanth and the lungfishes and, of course, there are millions of different kinds of invertebrates that pre-date any fish!
-Joe