Ahhh fishing, soooo many hours fishing.
I'm currently level 50 fishing, and as to this point, I have not found a more profitable spot than the small pond at the bottom of the cliff in the SE corner of La Theine Plateau.
Yes, you do fish up crabs and fish here, but they should be too weak to you.
Head to San'd Oria and in Southern San'd Oria, east of the AH, you will find Lusane.
Lusane sells little worms for 3 gil a piece if your fame is high.
Buy a comp fishing rod, and a bunch of little worms.
Head to that pond I mentioned in La Theine Plateau and go to work.
In this pond you will fish up;
Crayfish > 500 gil a stack, sell very fast.
Moat Carp > 4,000 a stack, sell almost instantly.
Dark Bass > 1,000 a stack, sell very fast and will make your fishing skill rise all the way to 36.
Rusty Caps > Sell for 1K at AH.
Rusty Buckets > Sell for 51 gil to Lusane, or use them for Bastok Fame, or chuck them.
The fishing here is really good, but also, a Water Elemental spawns every time it rains, if you kill it fast enough you can get it to spawn 2 times every time it rains. If you can kill it, you are looking at 1 to 3 clusters every time it dies.
I believe water clusters are going for 700 a piece in San'd Oria.
Last but not least, make sure to walk back to San'd Oria
On the way, look for rams. Kill one on the way back, you can get Skins = 500gil Horns = 1,000.
Now while walking through West Ronfaure, make sure to pas by where Jaggedy-Eared Jack spawns. You never know, you could get lucky.
I've claimed him 3 times, never ever camped him, just passing through.
And yes, I did get a Rabbit Charm.
On a full moon, or new moon, I've pulled in about 100K in a 6 hour fishing day. (Heh, I can fish while I work, as my job involves working on computers)
But this was also when Rusty Caps were 2K a piece. before SE nerfed them.
Oh, and a little misconception, if you put rusty bucket in your mog house, you will get Mog Enhancement: Fishing.
Don't be confused, this will not increase the number of fish you catch, but it WILL increase the number of rusty items you fish up.
Good luck, aside from that and camping the Tumbling Truffle, farming his brother fungars and bees and bats, thats how I made gil until I was level 50.