Randy Holmes-Farley has done numerous studies on the grade of puckling lime and has found that the Food Grade generally has fewer heavy metals and in general is more consistent as opposed to the highly priced aquarium grade stuff.
He attributes this to the fact that the aquarium grade products are not regulated by anyone (neither is the lab grade stuff). The food grade product, however is manufactured by folks that have the FDA breathing down theior necks every day.
He uses the food grade Mrs. Wages or Ball's as do I... no issues with it at all and it is a tenth the cost
Do a search out on RC or Wet Web Media and you will find that more and more folks are using pickling lime with great results.