"Hum, I read you can pait aquarium ornaments with acrylics.Let it cure for a week or so and it should be safe. (ive never done this so I don't know.)"
Umm, just my two cents, but I have worked with acrylic paints, and most are non-toxic, but water-soluble. Therefore in the tank it could dissolve and come off, unless is were on a porous surface, which the tubing is not. Besides, acrylic tends to peel when its on nonporous surfaces, especially flexible ones.