This is a next version of an aquarium controller and monitor. The previous version is here
It controls a heater via an MQTT (Tasmota) WiFi smart plug, measures temperature with a DS18B20 waterproof probe, uses a PID algorithm to control the set temperature, logs current temperature to InfluxDB and activity status to MQTT.
include/config.h.sample needs to be copied to include/config.h and filled with values for your environment (WiFi, MQTT, InfluxDB, timezone).
Total $36 of parts see BOM
The MQTT broker runs on a Raspberry Pi used for other purposes. That isn't included in the cost. Any heater which will always be on when powered up from the wall socket will suffice. I used a 150W Fluval M heater for $35.