I built something like this but found that the electric arduino water sensor quickly became corroded after a week and a half and ended up replacing it with a float switch.
I also built something along these lines about three years ago and use it every year. I started off with a stainless steel hall effect float switch and it's lasted so far, on its fourth year. I don't have a pump for my water, I use my RO system as a source and a solenoid as a valve. I do use a peristaltic pump to meter out Christmas tree preservative according to the readings from a flow meter on the water side though. It's worked wonderfully, even when changing tree stands.
Make sure to read up or watch a YouTube video about which ones are trash, though, since they mostly look alike (and some can be solved with a bit of soldering).