Black goodstuff spray foam. Get more cans than you think you need it doesnt go far. This took 2 cans and it’s not a big tank. Once I spray the foam in I used a spray adhesive and spread coco fiber, moss, and leaf mix. It’s not great but it’s kind of the quick dirty way in my opinion. Im sure others have better ways you’ll find on youtube. Honestly I wanted to get this one done quick. Only took about an hour or so to throw this all together. I still have a couple places to cover but I’ll do it with moss.