Can you use は to mark the object with negative verbs?


I've been studying Japanese for about two years, and recently started working
though Japanese Duolingo. I'm confused by their particle usage for verbs:

> 魚を食べます

versus

> 魚は食べません

I.e using を for positive verbs and は for negative verbs. Is this standard, an
idiosyncrasy of Duolingo, or wrong?

