Last year I made a grow tower. From PVC pipe and a heat gun. I filled it with pea gravel and planted Basil in it. The Basil grew very well but after a short time became root bound.
I want to use the tower again but what media should I use?
Is it weight related and settling you think?
If so..try a media that grants more space for root travel. Bio-Balls (http://www.aquacave.com/15-Bioballs-25-gal-200-ct-by-Won-Brothers-P...), or fill the bottom with egg rocks or those middle sized pebbles (basically bigger media on bottom) then use your pea gravel on top for supporting your starts in whatever media your seedlings are in or instead of pea gravel a mix of vermiculite, perlite, & coco may provide a more sand/soil type support structure for the plant on top.
In those sort of towers, most anything that grows for very long or very big is going to get root bound so I'm not sure it matters that much what the media is, if you leave the plants too long the problem will happen again. Choose a media that won't clog too easily and cause overflow problems and choose one your don't mind handling and/or replacing since you will likely have to dump and re-plant those towers each season you want to use them.