By Coco you refer to Coconut fiber? If you do, then I'm all with it. It's safe, natural, and only needs to be refreshed once a month. For water, just use a very large water bowl, or an aquarium divider sealed with aquarium grade silicone sealant.

If you do decide to go with water, use large and optionally flat pond stones for the very bottom, which are too big to be eaten. I don't recommend gravel, it can be ingested by the smallest of frogs, let alone an African bullfrog. I recommend a strong filter, like a canister filter or Fluval U underwater series.