A O Smith — Foul smell in water

I purchased a AO Smith water heater in 2019. The water till now smells very bad. The technician says we need to clean the tank which would cost Rs. 500 and it needs to be done once in six months. In that case it is Rs. 1000 per year. I'm using other brand water heater for 10 years in the same house and it doesn't cause any issue untill now. I think the company has to do something about this.