Two tanks are growing duckweed. Unfortunately the winter months don’t provide a lot of sunlight but it should be good for running process tests. The STM8S has been chosen for development. Raspberry Pi and Arduino are not conducive to low power solar operation. The STM8S has a low power sleep mode and is 8 bit which will be fine for this application. It is currently controlling a 5 volt, 55 milliamp water pump to provide periodic water circulation.
http://www.st.com/web/en/catalog/tools/FM116/SC959/SS1532/PF252276?sc=stm8svldiscovery This board can be bought for about $8.