The word "undercut" is thrown around like a fish at the docks... =P
It's called supply and demand If prices get high of course everyone is gunna start farming and putting it up for sale and if you want your stuff to sell you gotta lower the price to get it sold... It's a big hard world baby =P
You will only get Yellow Globes (no annoying Bastore Sardines) and can catch up to three at a time with the Sabiki Rig. I got 8 stacks in 1 hour.
Release all (!!!) and if the stamina drains quicker than usual, you got some rusty leggings (still sell at 800g per on Asura). Stand on the ice in the middle of the pond to avoid aggro.
The setup for these IMO: halcyon rod with worm lure, I got my worm lure from the fishing guild in Port Windurst for about 300 gil. Best place to fish would be in Port Jeuno where you board the Bastok airship. The worm lure will get you mostly yellow globes with the occasional cobalt jellyfish, and some rusty buckets. Once your inventory is full of buckets, hop the airship to Bastok and trade them into Foss in Bastok Markets F-7 (5 at a time) for 300 gil and some fame. Then you can drop your stacks of yellow globe in the AH for cheaper AH tax than Jeuno's, and do the airship passenger quest on the way back to pay for your airship fees. These are also great once you cap your fishing skill off of moat carp and need to reach skill 17 to start fishing on nebimonites. With a worm lure, you don't have to worry about buying bait so these can be really good for gil. Best of luck!
On my server the stock fluctuates between 0 and 10 regularly, but it's usually one buyer getting 5+ stacks at a time. The demand on this item is erratic, but high. Price it high, and it WILL sell!
It's awesome to see people load stacks upon stacks of these fish on ah knowing that only 3-7 stacks sell a day. 30 stacks on ah at one time is simply ridiculous.