HI,
I took some idea's I had and I have 10 water bottles we use for Dist water to drink and made a simple auto water refil system, simple to use and works great.
First I ordered a water level sense device their are two on the market that are resonable I decided to use the water float system it does minor water adjustments without major changes in level thats important for your sump skimmers to prevent daily tweaking.
Link to water float switch
Ultralife Float Switch (Saltwater Aquarium Supplies > Dosing Pumps/Top-Off > Electric Powered > Plug-in Units )
it was the most expensive but equally important to control level
I then took a 5 gal water jug that I use for drinking water its the clear blue type.
I used a simple air hose tubing that was 25 ft , and cut it down to around 15 ft so I could hide the bottle
I punched a small hole in the cap and I put a one way stop valve over the 5 gal bottle this hold the tubing at the right lenght in the bottle and prevents backwards siphon.
cost a buck
I then used a vaccum lift pump they use to siphon air out of those over flow tubes its also used to pump water to reptile cages for rain effects it only can handle around 3 gal an hour no problem cause i only need about a quart or so to top off the sump
Link to pump
Overflow Boxes
this pump is controlled buy the power supply on the float switch.
one side is air/water the other is vaccum in put. its designed to pump water and will probably last longer only pumping di water.
I put the pump on a hook hanging about 1 foot attached to the wood stand over the sump this will prevent siphoning if the power is off. beings the pump only moves around 3 gal an hour its very slow to overflow so far the switch has worked flawlessly. I also wrapped the end of the air hose to the larger skimmer in the sump so I can see of water is running.
the only other item I uses was a anti-siphon air line water stop valve to reduce the water return to the bottle this will make it easier for the pump to pull water and will require less time to refil the sump it only runs about 20 secs and the float turns it off.
its that white post between the skimmers.
I am amazed how simple and efficient this system is I may have about 60 bucks tied up in the system beings I owned the bottle.
this will prevent water needs for my system to about 1 week replacing the empty water bottle and thats it,
I would'nt use it for caustic material like Kalkwasser because it would be hard on the pump that may only use simple rubber valves but it has greatly reduced my daily maint and sump tuneing of the two skimmers they are skimming 24/7, do to a very stable water height. and the spec grav is very stable now.
So this was well worth the money invested for me and I marked the water level and its holding perfectly,
I will update this post as problems arrive.
Thanks for looking.
Doug
psps My two tanks 37 gal tall and a 55gal OP tank they .share a sump with a shoe box reforguim with algae that has mud, in a 20 ga tall tank with 5 inches of sand that has a small 2 fish phos reactor and a 65 gal coral life and a 220 gal coral life skimmer ( I bought the second one after the smaller one and hated to retire the 65 gal it work flawlessly. i also have a tungston heater in the sump with about 30 snails , beings I have two over flows to the tank and two return pumps I seem to evap alot of water almost a gal a day and in the winter month its get worse with extra heat and dry air. so this was never ending pain to refil the sump daily and tune the skimmers I would often over fill the sump and power down the pumps this causes a small over flow if I was sloppy, now the water lever is constant and at the correct heght for skimmer function. No more listening to pumps wheezing due to low water cause I skipped a day to refil the sump.
WHOOO..
Side note it uses less water than I did manually I may have been over filling the sump losing water to over flow and skimmer being to high filling the larger cup it holds alot.
psps DOn't tell amcaring about my good Kitty She'll want one toooo.
Bargin basement water top off 
