Gst registered & turnover < threshold

Basic question:

I understand that If turnover < threshold 20L annual turnover, no need to register with GST. But I already migrated to GST and turnover far less than threshold. Now -

1) should I add GST to clients and deposit to govt. via GSTR-3B  (even when turnover < 20L)

OR

2) not add GST to clients & file Nil return for GSTR-3B? ( until my turnover reaches 20L)

Please clear this doubt.

Thx