Use a float from a swamp cooler and use a solonoide (electrical valve ) on a timer. Worked for me over 20 years.
I always filled at 3 am and turned on the tank mixing pump for 1/2 hr after refill. Also ran the mixing pump for 10 minutes before a system pot fill.
This was a 100 gal res. and failure was out out of the question.
Note very rare is a 3 am power failure.