It'll be quite easy to get her to replace that sea bass, because its tank is a 1 by 2 tile item. This means that it can only be replaced by another item that also takes up two tiles. This is good news because if you gift her a new item of that size, it can't replace any of the items in her house that have a different footprint.
The image below should look vaguely familiar if I tell you that it's Reneigh's house. The boxes show you all the items she has, or where she could put new ones. This image was based on the game code, and it shows us that there are only two spots in her house that fit a 1x2 item: the one near the door where she put the sea bass, or the one in the middle of the room where the table is. She can't shuffle furniture around and make room, the locations are fixed.
View attachment 351157
(Source:
https://nookstop.ssmvc.org/house.html?id=hrs16)
As long as the item you give is the right size, something like a couch or a counter or a rectangular table, the only risk is that she
might replace the table. But if she does do that, you can give her another one of the same item and she until she replaces the sea bass with that. And then you can fix the table by gifting her a new one. She won't be able to replace any of the other items. It's safe!