Most will tell you you want 0 stray voltage. This is the best case scenario. That being said I currently have about 9 volts and my tank is very healthy and thriving. I have a mix of SPS, LPS, Softies, and anemones. I have encountered many other tanks with a similar voltage that are doing well. My problem is that i have anywhere from .3 to .8volts coming from multiple powerheads, heaters, and pumps. I refuse to replace them all. If I had one piece of equipment with 9 volts coming from it I would replace it in a heartbeat. Set your voltmeter to the 0 to 200 AC voltage range.