by I don't want to go too in depth here, because fishing is about discovering new fish and locations for yourself, that's really the whole idea and I don't wanna spoil the fun for you all 
. What I will tell you is which fish are able to be used as mooch, so that you can better discern what to aim for when you're trying to unlock the last few fish and can't figure out if they're mooched by a previous fish on your fishing log or not.
Please be aware that to mooch a fish you will need the HQ version of the fishes listed below. Double mooching requires a double proc of HQ (very rare) so you will need a very high perception. And there are legendary fish that do require double mooching to land. I suggest after 25 buying HQ pieces of gear and having a crafter attach perception materia to your gear, then you can go fill out your previous zones with ease. I've got weaver to help you with this process, so if i'm on you can ask me if u want or just send the gear to me via moogle mail and I'll attach it for you and send it back^^.
So far there is truly only 1 stat that affects fishing, and that is perception. Gathering, GP, and even DEX don't help as u A) don't use GP at all and B) Dex improves gathering proc % (and so does Gathering attribute), but you are already @ 100% proc rate. So again the only truly important stat to fishing is perception, which improves your HQ rate. Supposedly DEX is supposed to do something else for fishing, but so far all the sites I've checked say it improves gathering, and I haven't tested it thoroughly myself yet to discern otherwise, so we'll have to take other people's word for now.
*This is a work in progress and I will update as i discover new fish, but you can help me too by posting your finds in the comments below, just post any mooch fishes I missed, not what you caught with the mooched fish (don't want spoilers):
. What I will tell you is which fish are able to be used as mooch, so that you can better discern what to aim for when you're trying to unlock the last few fish and can't figure out if they're mooched by a previous fish on your fishing log or not. Please be aware that to mooch a fish you will need the HQ version of the fishes listed below. Double mooching requires a double proc of HQ (very rare) so you will need a very high perception. And there are legendary fish that do require double mooching to land. I suggest after 25 buying HQ pieces of gear and having a crafter attach perception materia to your gear, then you can go fill out your previous zones with ease. I've got weaver to help you with this process, so if i'm on you can ask me if u want or just send the gear to me via moogle mail and I'll attach it for you and send it back^^.
So far there is truly only 1 stat that affects fishing, and that is perception. Gathering, GP, and even DEX don't help as u A) don't use GP at all and B) Dex improves gathering proc % (and so does Gathering attribute), but you are already @ 100% proc rate. So again the only truly important stat to fishing is perception, which improves your HQ rate. Supposedly DEX is supposed to do something else for fishing, but so far all the sites I've checked say it improves gathering, and I haven't tested it thoroughly myself yet to discern otherwise, so we'll have to take other people's word for now.
*This is a work in progress and I will update as i discover new fish, but you can help me too by posting your finds in the comments below, just post any mooch fishes I missed, not what you caught with the mooched fish (don't want spoilers):
- Harbor Herring
- Copperfish
- Ocean Cloud
- Merlthor Goby
- Striped Goby
- Storm Rider
- Silverfish
- Common Sculpin
- Ogre Barricuda
- Abalathian Smelt
- Cloud Cutter
- Wahoo
- Saucerfish
