I believe you can just put it in a dish for them to lick, but you should also dust crickets with it. Calcium can only be absorbed once it chemically bonds with phosphorous, something crickets contain in abundance. The reason you don't want phosphorous in the supplement is because it has to be the right cal-phos ratio. Crickets + phosphorous in the supplement would add too much phosphorous to the diet in relation to the amount of calcium.

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"Eagles may soar, but weasels don't get sucked into jet engines." - John Benfield

Leopard Geckos are nocturnal and do not need a large amount of D3...They can actually overdose on it and become very ill...Do not put it in his tank. You should be giving him pure calcium in his dish and on his food and put a vitamin suppliment on his food every couple days as well. If you don't want to waist a perfectly good container of D3 calcium you can sprinkle it on his food once every week or so.