I tried the spot mentioned below, south of the outpost in Passhow, and found it to be a very good spot.
I used a Carbon rod and Fly Lure, also as mentioned below, at level 9-10 fishing skill, during Earthsday and half of Watersday, and got two and a half stacks of Copper Frogs before I got bored and left.
Moon phase was around 90% waning gibbous.
I also fished up a few useless Crab and Leech monsters, but nothing else besides the Copper Frogs.
If you like to hoard crystals like I do for crafting, there's a Water Elemental that spawns very near the outpost, in sight of the good fishing spot. I didn't bother finding the Thunder Elemental during the corresponding weather, but that's probably not far from the spot either.
Edited, Sep 18th 2009 4:25am by EurydiceKitty