i did black sole along with the other fish that produce 6 crystals from ranks 35-50. its true, it takes a very long time past 45, but this was the easiest path for me and is waaay more efficient than farming dart frogs for hours. you can forget about buying dart frogs on the market, they are rare, expensive and you will need tons of them - this is pretty much true for any ingredients in the rank 45+ recipes. i was also able to turn a huge profit from all the crystals.