If the water source is run-off only, how long do you think it would take to fill again if you drained it? I know that would be a SWAG.

You could also trap or seine as much of any undesirables out while initiating a new stocking plan.

This would only be in efforts to minimize any downtime from your mom's ritual evening feedings.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."