Ona Gel The ONA Gel is one of Grow and Harvests most popular products. ONA has been destroying odours effectively for many years and is used in hydroponics and commercial applications worldwide. ONA is not only an excellent air freshener, it is a true odour neutralizer – effectively eliminating bad smells. Formulated using a complex formula of essential oils, and contains terpenes. As a result, ONA is very safe to use, is biodegradable and can be applied in almost any situation. Note – Use with an ONA Breeze fan unit for improved effectiveness in large areas.