You'll want the substrate to be about 4 inches to provide stability for the plants. Bed-A-Beast (or any cocoa fiber) is also really great for plants. You may want to skip the catfish. FBs are pretty eager eaters and they may make the fish a meal. You're better off with a filter system and a partial water change once or twice a week depending on how many toads you'll be keeping and the size of the water section in the tank.
You can also put Potho clippings directly in the water. They will root and grow like mad adding a very natural look to your tank as well as help filter the water. Not to mention they offer great hidding spots under the leaves for the toads!
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Heather 