With having a siphon as your overflow your best bet is to hook up the air tube from a powerhead to the top of the siphon or use an aqua lifter because if that bubble breaks the siphon then the return won't know any better then to keep filling your tank til your sump is dry and your floor flooded lol
But if u don't wanna tap the u-tube then u could use a straw or something and suck the air out but keep an eye on that siphon...especially if your power goes out cause the siphon will fail then that's why so many people drill the tanks now which if you are just starting I'd advise doing and scrap the siphon
Last night the tube coming from the overflow box fell and all the water emptied onto my floor.
Drill it! You've just experienced first hand why a drilled tank takes away a lot from the stress your already gonna have lol...nows the best time though since you have no livestock and it really is a lot easier then you may think or if you decide to pay someone its well worth it