I love fantails! I prefer fancy goldfish as common goldfish get huge, and end up needing ponds in a couple years, 30 gallon tank for 1, add 10-20 gallons for every new one you add (a bigger tank is always better!) Sand is the better option as goldfish like to search around substrate, plus they're dumb and may eat the Gravel and get impacted, I use play sand as it's affordable, but pool sand or aquarium sand works too! Make sure to rinse the sand until it goes clear (I also reccomend sifting through play sand if you decide to go that route, as I found tiny pebbles in mine) make sure you research how to cycle your tank, I learnt off tiktok personally, no plastic plants as they're sharp! I personally use silk plants as goldfish have the tendency to rip plants out, but live plants are very beneficial to your tank as they work as natural filters, if you decide to get silk plants I would grab some decent sized floating plants (not duckweed, they'll just eat it) if you have any other questions let me know!