Tan (0_0) is intended as an even cuter or affectionate variant of -chan. It evokes a small child's mispronunciation of that form of address, or baby talk similar to how, for example, a speaker of English might use "widdle" instead of "little" when speaking to a baby.
In Cantonese and Hakka, the surname is pronounced 'Chan' or 'Chun', while 'Tan' is used by Hokkien speakers.
