dear spiderpig

my intuition tells me that what you have is a combo of both frogs and toad poles, the smaller dark/black tads being frogs, with the paler being toads - though to be truthful i have only noticed the paler colour on the tadpoles when they are fully in their last stage of development before their legs push out.

Do you see any spawn anywhere else in the pond - toads lay their spawn in strings usually around submerged vegitation - wheras frogs lay a mass od spawn in one area.

hope this helps