Awesome user name lol. You can use gravel that can be found at any pet store or fish store. Many here will recommend gravel w/ a grain size of 2-4 mm, though that figure's not written in stone. I know of at least one other user here that has seen a lot of success using Eco-Complete for the gravel layer.
my not much else is a female betta ( is that pronounce bay-tah or bet-tah? or is it regional?) and 2 serpae tetras. used to be a larger group. Feeling bad and want to give them a lovely new home they deserve. I have a new Rena Filstar canister I know nothing about. And dont even have a tank yet, altho it will be 30,40 or 55 gallons.
I have Eco Complete over soil in 6 tanks (only one is a true NPT) and it's working beautifully. I had originally used the Eco Complete alone in one tank and reused it when I switched to having a soil layer. It's great to plant in and I like the look of it so I kept using it. I've never had any issues with it and it's heavy enough to easily keep the soil contained.
I have one tank with AquariumPlant's substrate and that works pretty well and is a lot less expensive than Eco Complete but it is a lighter weight so it can be harder to keep plants anchored and I used a thicker layer than I would of the Eco Complete to make certain the soil would stay down.