Not to bore you or anything, I read a book mainly on urchins and it stated that since most urchins don't have eyes, the tank size means nothing for crawling around space its only a certains size for their food ratio intake. so in english If you were walking on a treadmill and closed your eyes it would seem liek your walking normally. The urchin is the same way, they may recognize certain elements in your tank but every inch they travel is just like walking around for us, and a huge tank or small tank, they will still have the same amount of energy to travel the same distance they would in a large or small tank, just like on a treadmill, you can walk 10 miles on a treadmill or walk 10 miles outside... either way it's still 10 miles... lol there that sums that up, And it's really a debateable issue in weather urchins are hurt by being taken out of the water. Myne has been in the air many times and has never been hurt from it. But i havn't seen much about this so i'm not sure. I have 3 urchins living in my 30 gallon for about 2 years, and they all are doing great, so it depends on food issues. Some urchins like the longspine is a natural scavenger and will find its way even without corraline and seaweed, others like the pincusion eat alot but they dont always need alot of food once established. Your urchin might need alot of food for the first couple of months, but slowly ween it off, or it will become dependent on that much food, and if any problem arrises making it impossible for you to give it the same amount of food you used to, he will notice and be a little upset maybe losing some spines. urchins like routine so try and ween their routine slowly to whats best for you and the urchin without making him fat and big, and dependent on your money.
Again nitrates are said to hurt urchins at around 45 ppm, urchins can withstand more problems in the water than some think, a nitrate count of 5 is not going to make a difference to him. urchins can live comfortabley in water with ntrates up to 18ppm once past that they will just change their behavior and act in our terms "grumpy", once your nitrates round the corner to 45ppm they will start losing their spines, and in the long term die. but your problem sounds as if you didnt acclimate him long enough, urchins notice little changes fast, and even with high nitrates, low food amount, and enemies, they can be fine, but if anything changes slightly it hurts them. and here is the way to tell. IF your urchin has just been added to your tank and you see his tenatacles reaching for rocks and are past his body, hes fine and is now feeling his new surroundings, but if your urchins tentacles are all sucked in and remain this way, its because of shock from new water parameters and it wasnt a long enough acclimation. it's similar to getting a side ache, when it goes away your fine (reaching out tentacles) when it persists and it hurts alot burning your stomach muscles you stop because it hurts too much (the tentacles are in and its is trying to get to better water). so it if you noticed this, that could have been the reason for your urchins spine loss, the shock persisted and the urchin became stressed. but its good to hear your urchin is doing good,
another healing method some aquarists use, to heal their urchin, is to pick him up under water and lightly poke his exhisting spines, this will trigger the urchins danger alarm and he will stop focusing on the water parameters and instantly start to move his spines violently, ( he thinks its a predator) by taking his mind off of what he thinks to be an uncomfortable environment he will now realize hes in a place where a predator is, and that he needs to get away, after rubbing him (lightly) he will try and hide or get away. after he thinks the predator is gone he will start to eat alot of food (provided you need to give it to him on this excersize) he is trying to nourish his body so he can grow back spines. and i'll explain it's really very simple. If the urchin has lost spines it is well aware of it, and by making it feel vulnerable to predators it will trigger a response of eating because it feels in danger without its shelids, so it will hurry to try and make new ones. But don't repeat this more than once or twice a month until his spines are back, otherwise if yo do it too much it will become stressed. Thid method has been proven in a study to increase spine growth rate about 10% because they urchin doesnt want to be eaten obviously this is its instict. so if you do this to heal the urchin he will respond in a posotive way, but this is only if he is still losing spines, if he has stopped and is now behaving normally totally then dont bother with this as he is now feeling more comfortable and this might just aggitate him.
hope that helps its alot to take in.i have alot more if you want it. I can tell you how to increase his color without buying expensive food too n other things. good luck 
Again nitrates are said to hurt urchins at around 45 ppm, urchins can withstand more problems in the water than some think, a nitrate count of 5 is not going to make a difference to him. urchins can live comfortabley in water with ntrates up to 18ppm once past that they will just change their behavior and act in our terms "grumpy", once your nitrates round the corner to 45ppm they will start losing their spines, and in the long term die. but your problem sounds as if you didnt acclimate him long enough, urchins notice little changes fast, and even with high nitrates, low food amount, and enemies, they can be fine, but if anything changes slightly it hurts them. and here is the way to tell. IF your urchin has just been added to your tank and you see his tenatacles reaching for rocks and are past his body, hes fine and is now feeling his new surroundings, but if your urchins tentacles are all sucked in and remain this way, its because of shock from new water parameters and it wasnt a long enough acclimation. it's similar to getting a side ache, when it goes away your fine (reaching out tentacles) when it persists and it hurts alot burning your stomach muscles you stop because it hurts too much (the tentacles are in and its is trying to get to better water). so it if you noticed this, that could have been the reason for your urchins spine loss, the shock persisted and the urchin became stressed. but its good to hear your urchin is doing good,
another healing method some aquarists use, to heal their urchin, is to pick him up under water and lightly poke his exhisting spines, this will trigger the urchins danger alarm and he will stop focusing on the water parameters and instantly start to move his spines violently, ( he thinks its a predator) by taking his mind off of what he thinks to be an uncomfortable environment he will now realize hes in a place where a predator is, and that he needs to get away, after rubbing him (lightly) he will try and hide or get away. after he thinks the predator is gone he will start to eat alot of food (provided you need to give it to him on this excersize) he is trying to nourish his body so he can grow back spines. and i'll explain it's really very simple. If the urchin has lost spines it is well aware of it, and by making it feel vulnerable to predators it will trigger a response of eating because it feels in danger without its shelids, so it will hurry to try and make new ones. But don't repeat this more than once or twice a month until his spines are back, otherwise if yo do it too much it will become stressed. Thid method has been proven in a study to increase spine growth rate about 10% because they urchin doesnt want to be eaten obviously this is its instict. so if you do this to heal the urchin he will respond in a posotive way, but this is only if he is still losing spines, if he has stopped and is now behaving normally totally then dont bother with this as he is now feeling more comfortable and this might just aggitate him.
hope that helps its alot to take in.i have alot more if you want it. I can tell you how to increase his color without buying expensive food too n other things. good luck 
