180 will work for some rays. If you are going to do a 180 for a ray for life you have to shop for the right ray. First off do not buy a pup as you will have no idea what size it will turn out. The next thing is buy a ray that is at least 3 years old and has been in a proper home for this period. That will be the hard part as you will have to have total faith in the seller as to how it has grown up. I just sold a pearl x motoro pup that is two years old and smaller than my female scobina which is maybe 1.5 to 2 years old, all it's siblings kept in the same tank have grown huge so there are small healthy rays out there. For your situation I would reccomend growing out your favourite rays and then finding people to sell or trade to for smaller rays. Don't let the size of your tank stop you from enjoying rays.