The ability to fine-tune communication performance is key to meeting the requirements of Internet of Things applications. While years of low-power wireless research now allows developers to fully optimize the performance of applications built on top of IEEE 802.15.4, this has not yet happened with Bluetooth Low Energy (BLE), whose networking performance is still largely unexplored and whose potential is not yet fully exploited. Indeed, BLE radios are often treated as a black box, because they are meant to only execute data transfer commands and manufacturers build BLE soft devices with closed-source network stacks. As a result, developers working with BLE cannot modify the radio driver or the link-layer, and hence have no direct control over radio duty cycling and packet re-transmissions. To tackle these challenges, we analyze and model how specific BLE features can be used to fine-tune communication performance at run-time. We further present the design and implementation of BLEach, an IPv6-over-BLE stack that exposes tuning knobs for controlling the energy usage and timeliness of BLE transmissions and that allows to enforce a variety of quality-of-service (QoS) metrics. We design three exemplary modules for BLEach providing novel BLE functionality: adaptive radio duty cycling, IPv6-over-BLE traffic prioritization and multiplexing, as well as indirect link-quality monitoring. We integrate BLEach into Contiki and release its code, thus addressing the lack of a full-fledged open-source IPv6-over-BLE stack. Experiments demonstrate that BLEach is lightweight, interoperable with other standard-compliant devices, and reduces energy costs by up to 50 % while giving QoS guarantees by quickly adapting to changes in interference, traffic priority, and traffic load.