Polystyrene As Decor

I got some polystyrene packaging, and was considering making some cave decorations with it, if I use correct adhesive would this be safe in a tank?
 

The polystyrene should be safe as Matt says. Make sure the glue is fish safe as well. I know cyanoacrylate glue is fish safe but I don't know if it can be used on polystyrene. And of course silicone sealant - without any anti-mould etc additives - is fish safe but again I don't know if it will work on polystyrene
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Were you planning on leaving them the polystyrene colour or using a coating?  This link shows how this kind of thing is usually done.  Enjoy your project!

http://www.cichlid-forum.com/articles/diy_background.php

I used silicone to coat my DIY PVC caves in 2-4 mm gravel.  There are always bits falling off and because it begins curing quickly (and is so tacky the whole time) it was a case of doing a bit then waiting 24 hours to do the next bit.  Three caves took around a couple of weeks so make sure you have an area you can put aside for that period.  You definitely don't want to be moving it around as it makes a mess.  Personally I found silicone very difficult to work with.