You could outline some of the fish with the invisible thread to stabilize the center then stitch in the ditch on both inner & outer edges of the dark green. Stipling or cross hatch would work on the outer border although I think it would be cool to hand quilt or embroider some fish hooks all around it.
I could also see a fishing pole embroidered or appliqued on one side with a fishing line going up & into the tank.
Nice wall quilt. Creative idea.