Plastic cups in fish tank

[Anna94]

Can I use this plastic cup as a cave in my tank?

 

[DreaminginBlue]

Correct me if I'm wrong, but that looks like an infamous red solo cup to me.
While it is safe to put in the tank, make sure you rinse it first to remove any dust. Also, it will float, so you'll need to figure out how to weight it so it rests on the bottom. You could try leaving it intact and burying it in the substrate. You also could cut it in half, take some gravel out of your tank, let it dry, and then glue it on the outside of the cup using an aquarium-safe adhesive. This will make it look more natural, in addition to weighing in down. Hope this helps!

 

[DreaminginBlue]

Some helpful links:

Aquarium Safe Adhesives:
Database Of Aquarium Safe Silicone (Add Your Aquarium Safe Product) - General Aquaria Discussion - AlbertaAquatica

Cave Inspiration:
http://www.aquariumadvice.com/forums/f20/make-my-own-cave-out-of-plastic-bottle-352602.html