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.
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.