I have found that my goldfish totally ignore lettuce. I have heard the same from others too. If you are going to feed lettuce, feed romaine or other darker colored greens.
Make sure you rinse it REALLY well. If it were me Id tear it into little bite sized pieces. Goldies like peas a lot more though. You can get a bag of frozed peas (no salt added) and thaw a few in a cup. Then take the pea and squeeze the inside out in your tank and discard the shell.

Mine absolutly gobbled them up yesterday.
Good luck with the lettuce. Let us know how your fishies liked it and what kind you used.