Instead of inking the entire surface of the tank, use ink in the cracks and seams, and along the corners. Covering a large flat area with ink is difficult to get a uniform coat. One drop of soap in the bottle helps, but to really get a smooth flow you need to use a flow enhancer and thin with an acrylic thinner instead of water. You could try thinning your inks with a mix of 50/50 water and Future floor wax, but I prefer acrylic thinners made for that purpose. You can get them in any store that carries a selection of acrylic paints. Another option is to thin with alcohol instead of water.