IphoneX - Won't come on when not using my phone?? – Aquarium Live - Real Fish Tank Review

Have a IphoneX- Where is the setting to keep it on in the background?? Had on of my previous Samsung Note4 & it worked ALL the time when I want using it.. Only came on at times when I set my phone down for the period I wasn't using it.. it didn't stay in forever.. was A GREAT APP!!
Review by funkyones-FALLBROOK, CA on Aquarium Live - Real Fish Tank.