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. 

I chimed in on a similar post last month for someone who lost the Aquarium screensaver due to not having a 4K Roku device. 

This Roku screensaver appears to be the classic "Sachs Marine Aquarium" screensaver that Roku probably licensed, and was also licensed by Microsoft at one time. You can check out a version of it here:  https://www.youtube.com/watch?v=9ldeOT3Ysgs  

The video above is a non-branded version (No "Roku" emblazoned rock in the aquarium). Plus, it has the sound of the air bubbles that the Roku screensaver doesn't have.  You could run that off of YouTube or actually download/save the file as a video and put it on a USB stick to run it that way.  

It's a 2-hour video.