Semi automatic water changer

Wanted see if anyone sees anything wrong with this.

I want to make it so that I can just fill my 210 gallon tank from my 55 gallon drum and have the tank empty on it's own. My water level is consistent in my sump as I have a mag float connected to a reservoir for ATO. I am thinking about drilling a hole just above the water line and connecting a hose to it. My hopes are as I fill the tank from the drum, the water in the sump would automatically feed out into my yard.

Does anyone see any issues with this before I start drilling?

Thanks Brad

 

Sounds good, but I would worry about back flow from when your pump shuts off, you will lose water and your ato will compensate later, lowering salinity. I would put the drain at the water level of your dt

 

I had plan on putting a ball valve on my hose for water leaving the tank so that when I am done pumping in water, I could shut off the hose. Also, the hose exiting the sump would be higher than the end of the hose in my yard so I would think that this would reduce the chances of back flow. What are your thoughts? Thanks for the suggestions on this.