Please let me know specifically out of these items below, what is Ok for my freshwater aquarium and will not effect the water parameters? Obviously I would thoroughly clean each item.

- Drinking glass or glass jar
- styrofoam cup
- plastic margarine container

If you have any other ideas please feel free, thanks.

no plastic or styrophom, you can use shells dipped in boiling water, this will clean it and make it sutible for the fish, it would be best if you got a cave from the pet store or put in an opace drinking glass dipped in boling water. if you have a betta fish go here, it has lots of info on bettas http://thefishingblog.wordpress.com/

Glass is fine, but I would avoid the plastic cup and container. They can leach chemicals into the water over time. Unglazed ceramics such as plant pots are fine, though, as long as there are no sharp edges.