I was going to use Hydro-balls as a drainage layer in my 18"x18"x24" white's tree frog vivarium, but I saw that it was $10 for 2.5 lbs, and I knew that it would be expensive if I had to buy like 3 bags of it. I have 10 pounds of gravel that I was going to return and I am wondering if I could use this as a drainage layer rather than the hydro-balls. Does it work as well. I don't really care about the extra weight...its not like I am going to be carrying my vivarium around the neighborhood. Would you use gravel for drainage? Please...what are the pros and cons of gravel...or should I just get hydro-balls? Thanks.