What size is your curren tank? And summer isnt coming for months, I live in NY :c
I know youre waiting for those new tanks, but in the meantime, you can buy relativley cheap rubbermaid containers at any store in tons of sizes and use it as a temp. tank, that way you COULD get those heaters! :D Once you get the right tanks from the gold fish, you'd already have the heaters.
Marineland makes a good one for $18 or less with a Petsmart card, its a 10w. tetra makes a $15 one, but its got very mixed reviews.
While youre waiting, I would suggest not doing that water thing and instead opt for a heating pad on low under them (monitor it) or at least covering the top with seran wrap with holes in it and a blanket around it to keep the heat in.