Even if you cannot pay your full balance owing on or before April 30, 2018, you can avoid the late-filing penalty by filing your return on time.
If you owe tax for 2016 and do not file your return for 2016 on time, we will charge you a late-filing penalty. The penalty is 5% of your 2016 balance owing, plus 1% of your balance owing for each full month your return is late, to a maximum of 12 months.
If CRA already charged a late-filing penalty on your return for 2014, 2015, or 2016 your late-filing penalty for 2017 may be 10% of your 2017 balance owing, plus 2% of your 2017 balance owing for each full month your return is late, to a maximum of 20 months.
If you failed to report an amount on your return for 2017 and you also failed to report an amount on your return for 2014, 2015, or 2016, you may have to pay a federal and provincial or territorial repeated failure to report income penalty. If you did not report an amount of income of $500 or more for a tax year, it will be considered a failure to report income.
The federal and provincial or territorial penalties are each equal to the lesser of:
10% of the amount you failed to report on your return for 2017; and
50% of the difference between the understated tax (and/or overstated credits) related to the amount you failed to report and the amount of tax withheld related to the amount you failed to report.
To conclude ...
1) You should file your tax return on or before April 30 even if you can't pay your full balance.
2) Declare all your income to avoid heavy tax penalties that could go up to 50% of your unreported income.