security deposit is not allowed by law in Ontario. the only extra money a landlord can get when the lease is signed, is last month rental deposit and/or(maybe) some small amount deposit for the keys. if you are not sure, you can contact Ontario landlord tenant board for your tenants' right.
the rate for interest on last month deposit is the same as the rent increase guideline for the year, for now. before it was fixed at 6% which is very unfair to landlord