Heads up... private English teacher - extra money

I was a private English teacher in Murcia. One day, I got a call from the police who had found my advert online in "tusclasesparticulares" and other places... they asked if I was legal. I said I had a visa to be in Spain cause I'm British. They said OK, but I had no permission to work and not pay taxes on earnings. They gave me a warning and I got the "alta" to be legal. It was just 3 students in a pub, but they said it was illegal. Be careful in Spain with online adverts. With the crisis the police are online looking at adverts. And as a legal teacher I paid only 15% of my money in tax and got it all back in 3 months. 3 Americans were sent home that year and 11 British too. Heads up to all Erasmus in Murcia.