Assuming there is calcium carbide in the mix, I suppose acetylene would form. It seems unlikely to be an issue though... just based on its structure CaC2 is likely unstable and hard to form; there's no way that free Ca ions would react with organics in the tank to form it. The only way it could get into the tank is if it's in the dry mix already and I doubt that very much. The formation of calcium carbide requires a pure carbon source and LOTS of heat (CaC2 is an alkyne which means it has a triple bond in it, the formation of which requires lots of energy).
Other impurities were my biggest concern when I first started looking into this... I'm hoping there isn't much else in this mix besides the ingredient list
So far (after 12 hrs) everything is dandy! All I have right now is a long tipped anemone and it's as happy as a clam. An article by Randy Holmes-Farley reassured me a bit about impurities though. He says that lime will react with most other metals like copper and precipitate them out.
Water parameters are fine right now. pH is about 8.3, specific gravity is about 1.024, nitrates are near 0. Unfortunately I don't have a Ca test kit right now... that would be the most useful right now
I'm going to try to pick one up soon.