Thanks for the quick replies everyone. I just figured it out and you're right, it was the return backsiphoning. I'm a dumbass. Normally the return tube has holes at the top to draw air in which breaks the siphon when the pump is turned off, but I forgot that I played around with the tube awhile ago and forgot to reposition the tube correctly. Hard to explain but thanks! You guys are always helpful
I think my brain needs recharging...