Calcium is probably the single most overlooked element in isopod care. People get the substrate right, they sort out humidity, they remember to add leaf litter, and then they forget that these animals are crustaceans with a calcium carbonate exoskeleton that needs rebuilding after every moult. Without enough available calcium, moults fail. Failed moults kill isopods. It's as straightforward as that.
Why isopods need so much calcium
Unlike insects (which have chitin-based exoskeletons that are comparatively light on minerals), isopods have heavily calcified exoskeletons. They're crustaceans, more closely related to crabs and shrimp than to anything with six legs, and they moult their exoskeleton in two stages. The back half comes off first, then the front half a day or two later. During this process they need to rapidly harden the new exoskeleton, and that takes calcium.
Isopods are actually quite clever about calcium management. They store calcium in specialised structures called sternal deposits (sometimes called sternal plates or calcium bodies) in the days before a moult, building up reserves they can draw on once the old exoskeleton comes away. But these reserves can only be built if there's calcium available in the diet. If the cupboard's bare, the reserves don't get stocked, and the moult goes wrong.
Females have an even higher demand. Brooding females carry developing mancae in their marsupium, and those offspring need calcium for their own tiny exoskeletons. A calcium-starved breeding colony will produce fewer young, and those that do emerge tend to be smaller and more fragile.
Cuttlebone
Cuttlebone (the internal shell of cuttlefish, Sepia spp.) is the most popular calcium source in the hobby, and for good reason. It's almost pure calcium carbonate, it's cheap, it's easy to find (any pet shop with a bird section stocks it), and isopods can rasp at it directly. Just snap a piece off and drop it in the enclosure. They'll slowly consume it over weeks.
One tip: break it into smaller pieces rather than leaving a whole cuttlebone in a small tub. More surface area means more access, especially for mancae who can't get much purchase on a large, smooth piece. The soft, crumbly side of the cuttlebone is easier for small isopods to eat than the hard outer surface.
Crushed eggshell
Eggshell is also mostly calcium carbonate and works well as a supplement. Rinse the shells, let them dry completely, then crush them into small pieces. Some people bake them briefly at a low temperature (around 100C for 10 minutes) to sterilise, which is a reasonable precaution.
Eggshell breaks down more slowly than cuttlebone in the enclosure, which can be a benefit because it lasts longer. The downside is that very small pieces can get lost in the substrate, so it's harder to tell when you need to add more. I tend to use eggshell as a background supplement mixed into the substrate and cuttlebone as the primary visible source.
Limestone and calcium-rich rocks
Natural limestone can be placed in the enclosure as a long-term calcium source. Isopods will graze on it gradually. The advantage is that it's essentially permanent. A piece of limestone in an isopod tub will outlast the colony by decades. The disadvantage is that it releases calcium slowly, so it works better as a supplement to something more readily available like cuttlebone rather than as the sole source.
Crushed oyster shell (sold for poultry) is another option. It's cheap in bulk and provides calcium carbonate in a form that breaks down at a reasonable rate.
Things to avoid
Calcium supplements designed for reptiles (calcium powder, liquid calcium) aren't ideal for isopods. The powder gets soggy and moulds in humid conditions, and liquid supplements are designed to be dusted on food or added to water, not left in a substrate. Stick with solid, natural sources that the isopods can access on their own terms.
Don't use chalk or plaster. While these contain calcium, they may also contain binders, additives, or contaminants that aren't great for invertebrates. There's no reason to use them when cuttlebone and eggshell are readily available and safe.
How much and how often
There's no precise formula here. Keep a piece of cuttlebone in the enclosure at all times and replace it when it's been consumed. For a standard colony tub, a piece roughly the size of a 50p coin is enough to last a couple of weeks. Larger colonies go through it faster, obviously.
If you're finding that your cuttlebone disappears quickly, that's a good sign. It means your isopods are actively using it. Just keep it topped up. Running out of calcium for even a few days during a period when multiple animals are moulting can cause problems you won't notice until you find dead isopods stuck in half-shed exoskeletons.
Some keepers also mix powdered calcium (food-grade calcium carbonate) directly into the substrate when they first set up an enclosure. This provides a baseline level throughout the soil that isopods can access as they burrow. Combined with surface cuttlebone, it's a belt-and-braces approach that works well.
You can see the difference
Colonies with good calcium access are visibly different from those without. The exoskeletons look harder and more opaque, dead moults (the shed exoskeletons you find in the enclosure) are firm rather than papery, and mancae survival is higher. If you've ever had a colony that just seemed to stop growing despite everything else being right, check your calcium provision. It's often the missing piece.