If you build the 5 gallon bucket with a toilet float valve aned 3-4 nipples on the bottom, attach it to a garden hose (you can buy a short (12") hose near the toilet floats that is normally used to connect the float to the water supply pipe in the houes. Basically you need to make the transition from whatever the fitting size on the float is, to the 3/4 garden hose.
Once you have it attached to the garden hose you can either leave it on at a trickle so it refills as needed. Or you can set the float high enough to provide 1-2 day's worth of water and just turn the hose on to fill every few days.
I don't seem to have problems with scum in my buckets as the lids are on (but leave them loose so as NOT to form a vacuum if you want gravity feed for the nipples) and I add a bit of ACV every few days to a week. I think you could also add a bit of bleach to the water (like a tablespoon or so) to keep bacterial growth down. But other than not clogging the nipples...I don't worry too much about the water as chickens will drink the filthiest water I have ever seen. I'm not sure it's good for them but mine seem to suffer no harm from an occasional drink from water that is full of poop.
I'm also hoping that a bird bath immersion heater will keep it from freezing this winter.