hi there louis..
its not cause by the water top up..
your fish is infected with white spots/ich parasites that are already in your tank water..
do some read ups..
here are some links for you to read up on reefers experience with ich
http://www.sgreefclub.com/forum/index.php?showtopic=37103
http://www.sgreefclub.com/forum/index.php?showtopic=36549
http://www.sgreefclub.com/forum/index.php?showtopic=34140
http://www.sgreefclub.com/forum/index.php?showtopic=32001