Been beading little fish charms for about three years now and I've learned the hard way that species choice makes or breaks the project. The trick is picking somethig with a real color pattern you can translate into seed bead bands. Clownfish, koi, and trout work becuase their stripes and spots map cleanly onto brick stitch rows. Something like a mackerel with those wiggly tiger stripes looks amazing but will drive you up the wall.
My personal rule is to sketch the pattern on graph paper first, one square per bead. Ten minutes of drawing beats an hour of frogging back rows because the orange band landed in the wrong spot. Also think about which fish reads well at charm size. A yellowfin tuna is basially one big blue-gray shape, so I'd add a bright yellow finlet detail or it ends up looking like a generic gray blob.
One more thing that changed everything for me: size 11 seed beads for the body, size 15 for the fins if you can handle them. The fins come out delicate and the fish stops looking like a pixelated blob. If you want a challenge after the koi, try an angelfish. The tall triangular silhouette forces you to learn decreasing on both edges at once, but once you've got it, whole new shapes open up. Pufferfish are fun too since you can bump the belly out with a few extra rows and it still reads as a fish.