Re: Rocks, Bungies, or Other
Well unless you want to get another tank that has a correct lid then your options are limited. I have seen people use weights and bungee cords (which look a bit obnoxious to me).
Re: Rocks, Bungies, or Other
i would not use rocks.
Personally, i'd return it and get a better working system. If you absolutely are set on keeping it, use bungee cords. They're safer and cant really be moved like rocks can. (I have cats and housemates so i would NEVER trust something someone else can move). The bungees are ok, just be very careful with them and watch your eyes. They can and do snap back.