Medium(#6 round). Koi Water Brushes are designed for the easy of water whether you are using watercolour paints, crayons, or watercolour pens, even when you're away from home or the studio. The brush separates into two parts: a fine flexible brush tip, and a barrel handle with a plug to carry the water. The comfortable grip is great for controlled blending, shading, and highlighting. The Koi Water Brush is available in three brush sizes.