What gives you the most trouble when completing a fish? Getting eyes to set correctly? Recreating scale patterns along the seams? Blending colors without over doing it? For me it's detailing fins with an air brush. Trying to get the rays to look like a fine line and to run straight. This usually causes me to over paint the fin, making it look darker than normal. I've been experimenting with fine ink pens, which seem to help in certain situations. Practice, practice practice!!!