Learn Italian: Il pesce è ancora vivo. - The fish is still alive.

The word ancora can mean "still," "again," or "yet" depending on the context. When it's paired with a continuous state like being alive (è vivo), it almost always translates to "still." In a negative sentence, it usually means "yet" (e.g., non è ancora... means "it is not yet...").

How would I say "The fish WAS still alive"? Should I use the passato prossimo or the imperfetto?

You would use the imperfetto: Il pesce era ancora vivo. Because being alive is an ongoing state or description rather than a punctual, completed event, the imperfetto is the correct past tense to use here.

Since pesce ends in -e, how do I know if it is masculine or feminine, and how do I make it plural?

Nouns ending in -e can be either masculine or feminine. You just have to learn them, but in this sentence, the masculine article il and the masculine adjective vivo give the gender away! To make any -e noun plural, change the ending to -i. "The fish are still alive" would be I pesci sono ancora vivi.

Does pesce mean the animal swimming in the water, or the food on my plate?

It means both! Just like in English, il pesce can refer to the living creature or the meat you eat for dinner. In this specific sentence, because it is vivo (alive), it's clearly the animal.