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.





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. you don't need a filter and can opt for daily changes, even with filters you will need to change water very frequently, have you seen the amount of poop they produce? lol in a rack system i would do coconut fiber as a substrate and water bowl big enough for a frog to move around and daily change it, that is how i keep mine, but not in a tub. You can't use just water as a substrate these are not aquatic species.
