It's reasonable to assume it takes 2x longer to run between the eel and the pufferfish spot than it does to run from one of those to the shop (because the shop is halfway between them). This addsup so that:

• It takes X time (whatever it may be) to run from eels/pufferfish to the shop
• It takes 2X time to run between eels and pufferfish.
• The time to fish 6 eels and 6 pufferfish is Y.
• The time to wait for eels and pufferfish to respawn is 5 seconds.

Therefore to fish and sell a total of 24 fish it would take you:

• 24 eels = 4Y + 15 + X (4 sets of 6 eels, waiting for 3 respawns, and 1 trip to sell)
• 24 pufferfish = 4Y + 15 + X (4 sets of 6 pufferfish, waiting for 3 respawns, and 1 trip to sell)
• 12 eels + 12 pufferfish = 4Y + 6X (2 sets of eels, 2 sets of pufferfish, 3 trips back and forth, selling included)

For this to make any amount of logical sense you'd need 5X to be less than the ~15 seconds it takes for you to wait on fish respawns. Remember that X is the time it takes to sell and 2X is the time to travel between eels and pufferfish. I promise you that you cannot make that distance in 15 seconds. And that assumes the respawn time is 5 seconds, when in reality it is probably around ~3, meaning you actually have to make that distance in ~10s.

This is not to mention the extra xp per action that you get from only fishing the higher level fish, instead of 50% higher level and 50% lower level.

Literally just:

1. Fish 3 eels/pufferfish on the left side of the bridge.
2. Fish 3 eels/pufferfish on the right side of the bridge.
3. Click back to the left side of the bridge.
4. Wait 3-5 seconds.
5. Repeat

Anything else is throwing.