Capital gains tax is due on your gain, so that's one BTC in your case.
No it isn't.
Okay so i understand that all bitcoin i receive is taxed as income and then taxed for capital gain once i sell but say this happens- I loan 10 bitcoins out and then at the end of the month i get 11 bitcoins back with the 10 being my principal and the extra 1 as interest. would i only need to pay taxes for the 1 bitcoin since i payed taxes already on my other 10 or is there something wrong with my assumptions?
That extra 1 BTC is interest income. It's just like being paid cash interest. Find the daily price on the day you received it and report it as income, unless the borrower sends you a 1099-INT, in which case you should use that figure.