Advice? Setting up 50g cube and sump

We recently purchased the SCA 50 gallon Cube. This is a big step up from our 21 gallon TruAqua Nano. (which has never worked right). The more I read, the more confusing it gets. I would like to start by setting up the tank as it was planned. And, work on modifications (such as a refugium) later when we understand what we are doing. It comes with no instructions. We have no experience with a sump and would like "guidance". The lights have been going out on the nano and the corals are looking sad. There is also a fair amount of hair algae on the rock and snails. Advice on moving the 21 to the 50? The coral on the rock? The rocK? We have purchased new sand. Any and all input is greatly appreciated

 

Hadn't thought of checking for leaks. Too trusting . Or basic flies over my head. Actually, most of our concerns are with sump set up. We've never had a sump and what I'm finding online differs greatly from what the tank comes supplied with. ( https://www.amazon.com/Starfire-Aquariums-Complete-Package-24x24x20/dp/B00HY5PD56 ) Eventually, I'd like a refugium, but to start, I'd like to stick with what came with the tank and learn as we go. thank you!