The only time you could reach 999 of an item is Either mega grind of constant New Game+
Kinda useless honestly.
It's actually quite easy to reach the 99 cap for seaweed and small fish.
Just go fishing with no bait, turn on an auto clicker at 1 click per 100 milliseconds, and go do something else for 20+ minutes.
(This is a fantastic way to train the Senses stat, especially if you undergo productivity hypnosis first.)