If you are selecting reptile supplements the choices broadly begin with on-food or in-water.
Now generally speaking it is always better to use on-food if possible because you can provide a broader range of vitamins and minerals that way. And typically on-food is a much more certain way to deliver minerals and supplements than in-water because water can get splashed around and spilt by some reptiles.
Reptile supplies: On food we generally recommend the higher calcium Nutrobal for younger growing animals and Arkvits for those which have reached close to adult size. Many adult animals especially benefit from the additional levels of vitamins A, C & E which we put into Arkvits.
You can use both Nutrobal and Arkvits together - it is not an either/or choice.
If you are stocking up on reptile supplies it's also sensible to use gut-loading insect foods Bug Grub or Grub Grub.
BSP vitamin drops and Zolcal D provide a way of supplementing animals via their water if for whatever reason the food route is not possible. When you use these supplements, do try to use them on alternate days rather than mixing them in the water bowl.
The reason for this is Calcium in drinking water can bind other minerals or vitamins and prevent absorption.