The "dead fish" classification is a fairly arbitrary choice of the steemitboard site. From the perspective of the "raw blockchain" all accounts are equally good, of course, and in practice not every account that has been inactive for 7 days should be really considered "dead".

@dantheman, for example, is a pretty active account here in general with nearly 7K followers. However, as it has not posted for a couple of months it is currently classed as a "dead fish" by steemitboard.