There are many dyes for PVC. Most are added or mixed in before melting or added at the melt stage, and then pelletized or made into a final product. But, even so, they are used mainly in rigid PVC, they have terrible lightfastness, and they tend to migrate. All in all a poor choice for the application and even worse for flexible PVC.
That being said, for your application, I suppost you could dissolve an AZO or Anthraquinone in a suitable solvent, then stain the "outside" of your aquarium tube.
It would be tricky. Sol Blue 35, Sol Yellow 16, Sol Red 135, might work. I just would not
recommend this as a feasible method for coloring clear PVC pipe.