Tatta / Tata (tah-tah) can be used as nicknames for Tatiyana but I don't know if they are exclusively - not commmon
Tanyusha (more popular than Tanusha but less so than Tanechka), Tanusha, Tanechka (the most common) and Tanichka (not common) are all "pet" forms of
Tanya (also
Tonya)
Tatiana can also be spelt Tatiyana (popular),
Tatyana and Tatjyana
Hope this helps :)
Alice