Abrupt changes In Temperature. Nothing serious. I would toss that grapefruit

Here is the tree... It does have scale. I am treating it with neem oil occasionally.

The temp in the room did go down to the high 50's at night during a cold snap. I generally lower the temp to the 60's every night.

Splitting is a symptom of Calcium deficiency and/or inconsistencies in watering.

I do allow the tree to dry out between waterings, perhaps too much. And with a cold snap during the holidays, it all makes sense. Just one fruit so far. I will keep the soil more moist and room warmer. I use a weak Foliage Pro fertilizer solution in the winter.

Related to Johnmerr's above post concerning calcium deficiencies, I should have mentioned that a foliar spray of Calcium nitrate during the trees flowering process reduces splitting in all cultivars, and especially mandarins.