Fthrow to Bouncing Fish isn't really an optimal combo, just a safe option that always works. For you to do optimal followups, you need to read/follow DI.
On Fast fallers:
If they DI Fthrow in, you can string:
3 fairs -> ftilts-> pp or dtilt -> more tilts to/or regrab -> anything else (usually bouncing fish to be safe)
2 fairs -> Ftilts -> dtilt/ pp -> more ftilts -> regrab -> anything else (usually bouncing fish)
If they DI Fthrow away:
It's still possible to string fairs to regrab but it's harder, and it's just more convenient to do Fthrow -> Bouncing Fish
If you notice their tendency of DI'ing away at low %, then it's better to do Dthrow. You can do things like:
Dthrow -> Fairs -> ftilts -> dtilt to/or regrab -> whatever
These are just on fast fallers though, against other chars you either do like Fthrow bouncing fish (or fthro/dthrow -> up smash), or try to be more creative with strings. Don't always do what's guaranteed, experiment other options.
Check these out:
https://www.youtube.com/watch?v=cpgYVgkXT9o&index=5
https://www.youtube.com/watch?v=totbtSToSFM