PVC works, I'd just make a cave outta flat rocks that is a little bigger than the knife. If you're willing to take a little risk (scratches) lean a flat rock up against the front wall near the corner of the tank that's bigger than the knife he'll hide in that and you'll be able to see him all day.
You could bleach rocks and then soak them in clean water and spray em' off in the shower. I just hose mine off outside, take them in and give them a good going over in the shower with very hot water. I have not had a problem with them yet. I got mine from a stream bed near my house.
You could bleach rocks and then soak them in clean water and spray em' off in the shower. I just hose mine off outside, take them in and give them a good going over in the shower with very hot water. I have not had a problem with them yet. I got mine from a stream bed near my house.