If you have a Serenitea Pot, then you've likely already learned just how fun it can be to decorate. While most furnishing blueprints can be bought or found in Remarkable Chests, some special items need to be acquired through other means. The Pool of Sapphire Grace is one of these items and lets you place fish in your Serenitea Pot.
The only way to get to the Pool of Sapphire Grace is by visiting Liyue's fishing association in Liyue Harbor. Luckily, the item is a cheap buy; you only need to trade ten Medaka to get the Pool of Sapphire Grace.
Once you've traded for it, you don't need to build it and can immediately place it in the outdoor area of your Serenitea Pot. Be warned; the pool has an extremely high load of 1,800. This will take up a good portion of the load limit in the area where the Pool of Sapphire Grace is placed.
You can place five ornamental fish in the Pool of Sapphire Grace at a time. Once placed in, you will see the fish swimming around in the pool.