to clean or not to clean

i just bought a new tank from a guy down the street and it looks like he never cleaned it.. the glass is covered with build up, i mean its pretty bad.
what should i use to clean the glass?? i tried scrubing the hesk out of it with a rough pad, but it caked on.
also the silicon looks a little rough, iam going to redo it. do i need to scrap it off or could i just put some more over it????
 
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