I was just wondering recently how this tank was doing; good to see that it's still a going concern.
My G.rhabdotus (kissing cousins to your terrapurpura) bred like mice outdoors last summer, and then spent the winter in an unheated basement tank at around 60-65F. Like yours, their metabolism slowed way down and growth was minimal. I don't think this is anything to worry about; it's likely the exact thing they experience in their native waters as the seasons change. Their adaptation to these changing temps is one of the main things about them that attracted me...and it's also why you can get away with keeping them outside all year, despite your terrifyingly frigid 40F weather...
I put my fish outside a bit too early, lulled into a false sense of security by a freak warm spell during spring. They wound up enduring almost a week in the 40F's, but bounced right back when things warmed up. I actually placed all my fry into a 350gallon stock tank, somewhere around 60 fish, and they are growing so fast now that you can practically watch them expand; they are mostly between 1.5 and 2.5 inches in size. Today I noticed some new fry in that tank; I suspect I will need to thin out the numbers before the end of summer.