What you've done is probably fine, but I encourage you not to plug that hole. The reason it exist is to prevent your aquarium from flooding over your floors.
You have a sump under your main tank that can only hold so much water. If your return pump loses power or fails, then a back flow will occur from the return nozzle. So say the power cuts - well due to gravity, water will inevitably be sucked into the sump from the display tank through that nozzle and potentially overfill and flood your home. This is known as siphon.
The hole that you've covered up will prevent this whole disaster because it allows air to travel through the hole at the height it has been drilled at therefore breaking the siphon aka "Siphon break." Without that hole, the main aquarium would drain all the way down until the very bottom of the nozzle. With the hole, it only drains to the very bottom of the hole, which in your case is none.
You can effectively see this in action if you were to hold that hole closed with your finger, and then turn off your return pump and watch the display tank drain a lot of water to your sump. The second you remove your finger, it stops the draining.
Edit: DCR beat me to it! They're gonna get all that sweet sweet "like" karma!