like Johnno I have a pipe, pvc, shaped a bit like the fill pipe below, to loop over tank so drains to exact requirements -while that is draining I vacuum with a 2nd hose.
for filling I use this:
fill pipe 1.jpg 37.88KB 16 downloads
outlet is just at desired water height - when I can't hear the splashing I know tank is full.
didn't think of the smoke alarm idea ... cool