well, anything that can suck up water can be used to drain the sump. i just use it to drain the sump when i need to service it. ie. remove it from under the stand. I have a 10 gallon and even then I have to empty the shopvac a couple of times before my sump is empty. Very handy.you can use it to drain a tank/sump? maybe I should get one just to do water changes.