Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
It should be in your devices Settings menu. Go to the list on the left & select Settings, then there should be an option for Screensaver & choose the one you want. The purple city (I think its cool) seems to be the default one now, but you can choose a new one for yours. FYI they occasionally 'push' holiday ones now & then so you might wake up once in a while on holiday (Valentines, 4th of July) to a temporary new one now & then. Personally I think its a nice touch.
You can also disable the Featured Themes in the themes menu if you don't want Roku to change them on you. To my understanding the aquarium is only available now on 4K devices. So your Ultra should still have it available. There are dozens of screensavers available, but note that some require a payment.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.