KH and GH out of my tap are both in between 2 and 3. It seems as it got colder both dropped one degree. Is that even possible?
It is, but more importantly are you testing your tap water when it's cold from the tap? If so, it might be your test kit reading wrong, they are calibrated for a specific temperature range. Too cold, or too hot, and it can be off.
It really is much easier to dose Calcium Chloride. It will be harder for you to get your GH adjusted using Calcium Carbonate, with CaCl it is much easier to measure the dry weight into a known quantity of water so you know the PPM of the dose bottle. You can then figure out exactly how much of the liquid CaCl to add to raise your tank water by the amount you want. Also CaCl is inexpensive, I think the last time I bought it I paid $5 for 2.2kg. So despite the fact you already bought the Calcium Carbonate it is not like it's an expensive mistake.
I also have to agree with Buck, I haven't used CaCl in my tank since I've had ferns and I've got pretty soft water, (GH 2) and the Java ferns have easily doubled in 3 months, and I've got a Bolbitus that's putting on mass like crazy. They seem more effected by nitrate, potassium, and iron IMO. All I fertilize with is a little bit of Kent Plant-Gro, which has all three.
One thing you could do, if your really stuck on using the Calcium Carbonate is premix some water for water changes and stop doing it straight from the tap. You can add the calcium to a 5-10g water container, with tap water and a powerhead until it breaks down, then change water with this premixed water, that way you don't have to wait for it to break down in the aquarium.