Microbial growth of at least one microorganism may be decreased, prevented, and/or inhibited in a fluid by providing ultrasonic waves to the fluid. In an optional non-limiting embodiment, an amount of at least one gas may be provided to the fluid for further decreasing the microbial population, preventing microbial growth, and/or inhibiting microbial growth. In another non-limiting embodiment, the fluid may be a downhole fluid, such as but not limited to, drilling fluids, completion fluids, production fluids, injection fluids, stimulation fluids, refinery fluids, servicing fluids, and combinations thereof. Alternatively, the fluid or downhole fluid may have a temperature ranging from about 25 C to about 100 C.