Consumers filed a class-action lawsuit Tuesday against Fitbit Inc. FIT, -0.15% saying that the company's heart rate monitors in its wristbands are inaccurate. In the suit, the complainants say the heart rate monitors on Fitbit's "Charge HR" and "Surge" products -- two iterations of the company's fitness wristbands -- were off by a "significant margin" and especially during periods of exercise. The proposed class action would include anyone who bought the Charge HR and Surge products from retailers other than Fitbit.com. Fitbit shares were down 15% Tuesday afternoon amid a daylong decline in the stock after unveiling a smartwatch that would put it into competition with Apple.