Welcome to the fourth guide in reef-pi series. In this guide, we'll be building a water level controller. In the home environment, reef aquariums lose water due to evaporation. The loss of water results in an increase in salinity since salt does not evaporate. Natural salt water has a fixed specific gravity (used to represent salinity) of 1.026. Corals are very sensitive to salinity and a small salinity fluctuation can cause significant stress to them. To address this, fresh water (filtered using reverse osmosis and de-ionized) is added to replenish the evaporated water, which in turn keep the overall water volume same, thus keeping the salinity fixed at 1.026. This is called top off.
While a reef hobbyist can manually top off, a water level controller automates the top off process by monitoring the reef aquarium water level and pouring fresh water by switching a pump. These types of controllers are popularly called as Auto Top Off or ATO. An ATO is critical for keeping salinity at exact 1.026 in smaller aquariums.
This guide assumes you have a working reef-pi setup along with the power controller module. Follow the first two guides if you have not. We'll be extending our previous power controller build to add an optical water level sensor and a water pump (connected via the American DJ SR P8 power strip) to pour fresh RO/DI (reverse osmosis, de-ionized) water into the aquarium. Although in this guide we are extending our temperature & power controller, it is also possible to build a standalone ATO controller using reef-pi.