Help with herbie emergency drain

[ragin_cajun]

I drain from the display tank. Do a 75% water change--when I do it. I'm doing it today just to get a nuisance convict out of a 540.

 

[FluffySackson]

But as your draining water from the sump doesnt the level in the tank eventually fall below your overflows, then only drain from your sump ? Forgive my ignorance im only just learning about sumps

Im envisioning the water in the tank droping a few inches until it gets lower than the overflow , the oeverflow loses siphon at that point only water from the sump is being drained. How would you do really large wc's ?

for 50% water changes, I just shut off the pump and change the main tank's water. And when you drain water from the sump with the pump on, the main tank's level is kept consistent until the pump runs dry.

 

[jaws7777]

Aaahh oh got it amd sorry for the derail.