like where can you use it or what is its recommended use? can you use it to make fishing faster for the achievements?
It can only increase item qty when you are picking up item.
In this game, most of useful items will be dropped from enemies so it's won't be affected by x25. That's why i recommended people not to use this if they are not familiar with this game. It can really ruin all your effort if you didn't use it carefully.
If you really want to use this, remember not to turn off it whenever you are going to pick up something related to
Story Quests / Side Quests.
I only find x25 useful when i'm farming Black Pearls in The 1000 Year Ark of the Ocean Palace and mining in The Time Mine and the Dreamers.