Jim,
I don't think a temp drop would bring on ick if the ick was not already in the tank. Ick has a specific life cycle. When they hatch and there are no fish to attach to, they die and there is no more ick. Something must have brought it into your tank over the last month or so and you may not have noticed the first or second cycles of adult ick on your fish.
I'm wondering if you might have picked it up from something that went into your tank during our frag and MA swap before Christmas. I had ick at that time, but I was VERY SURE I never took ANYTHING from my ick tank to pass along to anyone else. OMG, that would have been the worst thing I could have done!!!!, but was careful not to do that. My other two tanks are ick-free because I have been scrupulously keeping all cleaning/feeding tools, pails, etc. that I use for the ick tank away from my other two. Another PITA
To make sure you have no more ick in your ick tank, keep your fish in the H-tank for six weeks, leaving the ick tank without hosts for that period of time. Then your ick problems should be gone as long as you've successfully treated the ick in the H-tank.
BTW, how did you break you hydrometer? I broke mine awhile back, but was able to put it back together pretty easily. The H-thing dislodged and the swing thing came out. Yes, those are the technical terms, btw
I just put it back together, no harm done.