Comets and an 18 degree temp drop

[glassbird]

Here is my question...can comets handle a drop in water temperature of 18 degrees in 16 hours?

Details...
I have 9 comets that live in my pond during the warmer months and overwinter in my house. For the last three winters, they have been housed in a 29 gallon tank in my living room. This year, they all had an enormous growth spurt over the summer and when I brought them in and put them in the 29 gallon tank, I knew I had a problem...they were WAY to big for that amount of water. It took me a few months to find a solution and then come up with the money, but I purchased a 100 gallon Rubbermaid stock tank and set it up in my basement. I put a homemade filter and pump in, and let it run for a week. The water temp is between 46 and 48 degrees. It's not actually that cold down there, but the bottom of the tank is right on the concrete slab.

Two days ago, I began the process of moving the fish. I took 3 of them, put them in a 5 gallon bucket filled almost all the way up with water from their tank, and put them on a workbench in the basement. I wanted them up high on the theory that it might be a few degrees warmer there than on the floor. I added an air pump and air stone and turned off the light to (hopefully) minimize stress. Twelve hours later, their water temp had dropped from 64 to 52 degrees. Four hours after that, the temp was down to 48 degrees. I started slowly adding water from the stock tank and after about 2 more hours, I poured them into their new home.

I am in mid-process with the next three fish and the temps are very similar. That water in the stock tank is COLD. I have to put my arms in it to fiddle with the filter now and then. Am I asking for trouble with this temperature drop? What should I be doing differently? I should probably have investigated this BEFORE I started the move...

Is a temperature drop of 18 degrees in 16 hours potentially hazardous to their health?
Cathy T.