No mods. I caught three other legendary fish then came to the mountain lake and on the tiny island next to the one I fish on to get the Legend II, there were those sparkles in the water so I happened to have my pan on me and just collected them. Got the Legend II, but didn't "catch" for the achievement. I didn't notice, just thought I'd finished it (YEAH STARDEW VALLEY!) and went for the last one, Mutant Carp, caught it in a minute, then checked because I didn't get the achievement. The Legend II wasn't counted, so I ran back over there and fished. At 10 pm I made my post here looking for help, but didn't find any... so I went to bed. THis morning I woke up and tried fishing again. Caught a bunch of seaweed and carp and then .... DRUMROLL!!!! I caught the Legend II! So it was ALMOST MIDNIGHT in the game when I got it, not sure it would have counted any later! That was more excitement than I've had on the farm in a long time. SO IF YOU PAN during the Qi Legendary Fish II quest, check your achievements. I don't mod, but I was eating Seafoam Pudding (with the Qi Spice) and drinking Ginger Ale (also with Qi Seasoning!). So that bumped my fishing and luck up. (I didn't know until I cooked with it that it makes everything +1. Super helpful!) Anyway, wishing you all extra legendaries!