If you are buying and selling stocks using a TFSA you should not participate in day trading or overly frequent trading. Day trading is the act of buying and selling financial securities within a short time frame, with the goal of generating profit from trading activities.
If you do so, the government may consider your trading activity to be a business and will make you pay income tax on your trading activity. The CRA can audit taxpayers who actively trade using a TFSA.
Here is a link to a very interesting article in Financial Post
https://financialpost.com/personal-finance/stop-using-your-tfsa-to-frequently-trade-stocks-the-cra-may-see-it-as-business-income