As a scalper, you only want to trade the most liquid markets. These markets are usually in the major currency pairs, such as EUR/USD or USD/JPY. Also, depending on the currency pair, certain sessions may be much more liquid than others. Even though the forex markets are trading for 24 hours a day, the volume (https://www.investopedia.com/terms/v/volume.asp) is not the same at all times of the day. Another reason I trade with freshforex is, they have very low spreads of these two symbols