Sadat, blood tonic doesn't make you fat rather they help increase your appetite. To gain weight you just need to eat more high calorie and high carbohydrate content food such as most of our local foods(like rice,banku,fufu,waakye,ampesi,yam etc) while reducing the amount of exercise you do. This means doing less walking and running and if you play football or outdoor games that require you to run a lot you need to either stop or reduce the frequency in participating in those games. Have a clear idea of the weight figure you want to achieve like 60kg,75kg or even 120kg. This is to help you know when you get there so you manage the weight from there on so you don't gain more than you intend to.
PS: Eat heavier foods like rice, waakye, banku etc for your breakfast. not tea and others.