we should pay income tax i am right?
Any resident of any known country has to pay an income tax, where income means all incoming cash flows - no matter if it was a second hand TV sale or FX order result.
Then, it comes to your own responsibility and the extent to which you follow the national laws. If the latter is high, then each particular period of time (in most countries - each year) you have to pay back to the government a part of your annual income. Normally, if your incomes are low - competent governmental bodies are not interested in wasting time witnessing you and your operations - that's why majority of people on Earth ignores (at least - partly) this obligation. Another reason is that it's, frankly, impossible to control all cash flows in any country of a reasonable size.
It has nothing to do with Forex, it applies to everything you earn in your life as long as you live in a country. When you work for somebody (when you receive a salary), the tax is already paid by your employer - that's why in such cases you don't have to worry.