plural forms - Should I use hippopotami?

When should I replace an 'us' with an 'i'?

Are the following words valid?

Hippopotami (plural of hippopotamus)

Virii (plural of virus)

Bonii (plural of bonus)

When should I use 'i' and when should I just add es? Where does the i come from? And, when should I use 2 i's and when should I use just one? Thanks.