The Difference of The Particles "Wa" & "Ga" in Japanese?

Yes, sakana ga tabemasu and sakana WO tabemasu differ. The former means the fish ate, the latter that you ate fish.

Sakana ga and sakana wa, however, don't have much of a difference in that example.

Generally, though, wa points out the topic and ga the subject. A lot of the times they can be interchangeable, but there are some rules where wa or ga just always seems to be used.

For example, ___ ga suki desu, not ___ wa suki desu or ___ wo suki desu.

You (or at least I) wouldn't use "wa" in that example because when you're talking about what you like, you need emotion to it.

Ga gives the subject more of a punch in the sentence and is usually used when you're talking about something new in the conversation or when the subject is more important that the rest of what is said, whereas wa works with what has already been revealed or mentioned.