Depending on the kind of fish you give, vitamine B1 is needed everytime you feed fish. You can read this article about thiaminase which causes vitamine B1 deficiency and about which fish have it and which don't:
http://www.thamnophis.com/caresheets...?title=Thiamin,
Like James said you can also add some Calcium with it. It's interesting to give it with new-borns and pregnant snakes. If you don't give whole fish, it's necessary each time.
I would not use a general vitamine supplement every feeding, just about every 1,5 month or so.