Shaun asked me: Why do frogs need vitamins - they don't get them in the wild.
Actually, they do - but not out of a bottle. All living organisms need a range of vitamins, although they vary in what they can make and what they must take in through their diet. In the wild, frogs eat a much wider range of food items than the restricted range of insects we give them in captivity, and the wild feeder insects eat a wide range of foods, not just fly-mash, so wild frogs receive quite a lot through their diet.
Animals also need a range of minerals, one of the main ones being calcium (for bones and the nervous system). Insects don't have bones, and compared with vertebrates, they have a reversed calcium:phosphorus ratio, i.e. they contain more phosphorus than calcium, whereas in animals, it's the other way round. Most insects are therefore a poor source of calcium, although soil arthropods such as springtails and woodlice may contain reasonable amounts.
is needed to absorb calcium from the diet. In the wild, frogs make vitamin D3 in their skin when 7-dehydrocholesterol reacts with UVB ultraviolet light at wavelengths between 270–300 nm. Frogs kept under UVB illumination in captivity can make their own vitamin D3 - but they also need a wide range of other vitamins and minerals they can't get from a restricted diet, so we supplement their food.
Final point, vitamin D3 oxidizes and is destroyed very rapidly, particularly when damp or in solution. Check the date on your vitamin supplements - I have been supplied out of date vitamins by otherwise reputable suppliers which have had to be replaced. Keep supplements cool, dry and dark, exclude as much air as possible and don't keep them too long - if you don't use them up in 6 months, replace them, even if they are still within date. Once the pack has been opened, the decay begins and the expiry date assumes that they will be kept in a sealed package.