Dear Blue Lobster:
I'm afraid my electric blue lobster ins't getting enough calcium, what should I feed him?
Dear Gentle Sir:
Crustaceans form their exoskeletons from chitin, a leathery carbohydrate reinforced with calcium carbonate.
Without the calcium carbonate, as happens just after a molt, a crustacean is considered "soft-shelled." Some crustaceans eat their old shells to regain a measure of calcium so as to harden themselves. In an aquarium, where that might not be tenable, what is one to do?
One solution that was popular in the Eighties was to add milk to the aquarium. This was at the height of the American Dairy Council's "Does a Body Good" campaign, when the dairy industry was trying to create new markets. They introduced "Milk-for-Fish" to the pet trade in 1985.
Unfortunately for crustaceans in American aquaria, milk in the water had several undesirable side-effects. First, milk is slightly acid, which sent the pH levels of tanks off. Second, crustaceans can only absorb calcium by ingestion, so passing milk through their gills did nothing but make them breathe lactose and milk fats all day long.
Third, it made the aquarium opaque, so humans couldn't see their small friends and their small friends could not see one another. Fourth, it interrupted crustaceans' antennae's sensory abilities, causing them to bump into their environment drunkenly and miss out on food entirely. And fifth, it turned the water rancid and quite stinky.
After a backlash from the aquarium community, in which several lawsuits claiming false advertising were launched and several out-of-court settlements were made, the American Dairy Council withdrew their Milk-for-Fish campaign.
Since then, "calc-licks," small lumps of compressed industrial chitin by-product, have been the product of choice for crustaceans in need of a calcium carbonate boost. They can be had from your local pet store quite cheaply depending on the size.
You can also feel free to add washed shell pieces from your own shellfish dinners, something much cheaper and closer to what a crustacean might find in front of them in the wild.