There are dozen, maybe hundreds, of supplements out there for tortoises; for the most part, they're not needed if you're providing a varied and appropriate diet for your tort.
Living in New Hampshire with tropical and desert tortoises means that although I'd love to, I cannot always provide the perfect diet or environment year-round, so I am sometimes forced to supplement.
When needs must, I have chosen to make my own supplements, not because I like making my life trickier, but because I wasn't crazy about the options out there (by which I mean on Amazon or in my local pet store... the ingredients in the generally available supplements are legion, suspect, and often unknowable.
To combat those specific problems, I make mine with as few ingredients as possible, high-quality organic ingredients that I would use on myself, and comprised of simple ingredients that I can easily pronounce.
Challenge: Wintertime lack of weeds and flowers
The biggest challenge facing a keeper of tortoises, especially in NH, is that the weeds and flowers that my torts love to eat in the summer months are dead and/or under snow for much of the year
My answer to that is a mix of plant matter I know is great for any tortoises, which I shake over the mixed greens I feed my creep in the winter months; it's comprised of: moringa, echinacea, wakame, calendula, nettle, chamomile, raspberry, rosebuds, dandelion, hibiscus.
It's presence in their morning salads a few times a week increases the nutritional value of the greens they're eating by a lot.
Challenge: Calcium deficiency
Many of the plants they'd be eating in the wild contain more calcium than they get from the diet I can offer my tortoises year-round, so I need to supplement their calcium intake, especially with the females who may be laying eggs.
I offer my tortoises eggshells from local and organic eggs in two formats: whole and powdered. The whole eggshells are often as much a behavioral enrichment as a nutritional one, although I do see them disappear over time. The powdered eggshell is easier for them to metabolize and easy for me to insure that all of my torts get some by sprinkling a bit over their food 1-2 times a week.
Challenge: micronutrients, like iodine
There are some nutrients which are just tough to find outside of the animals natural habitat... iodine can be one of those. to deal with the lack of it in their diet I've actually heard of tortoise-keepers feeding their charges enriched bread or iodized salt.
I already had wakame, a dried seaweed, in my kitchen, for use in soups that I make from time to time. It's easy to rehydrate and serve mixed in with the greens that the torts get.
Challenge: neglect of rescue tortoises
I live with four tortoises that are rescues, and one of them in particular came from a living situation that involved poor feeding, low heat, and poor lighting, for years.
I made a "booster" that I added to their food a few times a week for the first year that they lived with me; it's comprised of a powder made from hibiscus (healthy and bright red), wakame (a seaweed), moringa (a superfood), and eggshells (for extra calcium).
Challenge: successful model syndrome (dehydration from living under lights)
Living in small enclosures, subjected to intense heat and light, and generally living in lower humidity conditions than would be found in their hides in the wild all combine to potentially dry out a tortoise in captivity. I have two solutions that I make use of from time to time.
I try to soak my sub-adult and adult tortoises in a warm (not hot) bath once a week (I feel that hatchlings benefit from a daily soaking for their whole first year); with healthy tortoises, I mostly just do warm water soaks, but if they've lost weight or if I have reason to think they're dehydrated or ill, in which case I sometimes soak them for a couple of hours (up to just above the line separating their plastron and carapace) in a rehydration bath made from powdered hibiscus flowers and a bit of cane sugar and sea salt.
When I soak and weigh and inspect my tortoises for their monthly health-check, I end the process with a rub-down of their shell with a shell conditioner that I made from coconut, olive, and almond oils. I don't know that they need it, but I think it makes their shells healthier in the artificial conditions under which they're living and despite what some old-timey keepers still profess, tortoises don't breathe or process UV radiation through their shells, so the treatment cannot possibly do them any harm.
The fine print
I should say at this point that I think it's entirely possible to keep tortoises for 50 years (or more) without using any supplements at all... a varied diet of appropriate and nutritious foods along with an environment that provides the proper amounts of heat and humidity and light are probably enough as tortoises are tough as nails.
That being said, I believe that the judicious, and sparing, use of the supplements I mentioned above, for the reasons I outlined above, can only help your tortoise live a happier and healthier life.
