Honestly, it looks like you're beginning to see some shell pitting or poor shell development because you're keeping them in less than ideal parameters.
While it can take a couple years for them to reach full adult size, you're likely inhibiting proper growth because you aren't providing proper parameters and are keeping the temperature way too low.
Just because you can keep something alive in a particular set of parameters doesn't mean you should. Especially when it comes to critters that aren't exactly common in the hobby. And when you could very easily bump your KH up and keep them in a tank with a heater with relative ease and little cost.