telling you your water is fine may be right may not be. If theyíre using strips youíre trusting that theyíve stored them well they are not too old, havenít gotten damp. Also many stores use strips that donít even have ammonia tests and thatís one of the major culprits.
Iíd do a 50% water change just in case. Wonít hurt; may help.
Then go back and find out how they are testing and the numerical result. If theyíre using strips Iíd recommend getting your own liquid test kit to confirm.