The general rule for fish in a thinner tank is to make sure the fish will not grow too long. Your fish should have at least a 1” space between them and the walls however 2 ½”+ is recommended. That means that if your tank is 6” deep you should have fish 3 ½” or smaller or if your tank is 8” deep your fish should be around 5 ½”. The good news is that most aquarium fish fall under this category. You can refer to these charts to help you choose.
6” tank depth http://www.walltanks.com/picture_library/medium_fish.pdf
8” tank depth http://www.walltanks.com/picture_library/large_fish.pdf