Perhaps I'm giving the fish entirely way too much credit here. I dunno., but here's my thoughts.

If we shove, we're only going to get the top of his range to call. Granted he's a fish, and the top of his range is much wider than a 18/16 reg. But I have a hard time including anything but Tx in his calling range that we beat vs a shove. Will he call with 99 or A8? Who knows.

If we check, he's going to bet all made flushes and probably check behind with everything else since he's so passive. I really don't mind a check/check here and getting a showdown OOP.

But what if we bet something like $6 (1/3 pot)? He's still going to move in with his made flushes. Since passive fish only re-raise with the nuts, and a smallish bet will get a call from Tx, A8, 99, etc, isn't this the best option?