you cant stop them from breeding unless you have all of one sex ''females'' as males will fight. You would have to cull alot of eggs and I mean a lot of them. My vent trios produced more than thirty froglets each a year inviv on there own.
As for temps I missed that you were that far up north. Im all the way down in the south east so heating is never required.
I know many great breeders up in Canada that could help you out as well if you need anything, There is
Canadart.org • Index page . Darts are growing in popularity up there and finding live plants shouldnt be hard I can help you out there as well if you need,
And make sure you cover the lid with a piece of glass as humidity will not stay in the viv and there is no way to keep a functioning viv for darts with out it. There are build threads on other boards on how to exactly do this.
Good luck,
Michael