Hi, I can tell you from my own experience with a pin cushion. They will not live long in your tank. I to have a 25 gal, I had my urchin for 7 months. My tank was over a year old when I got this urchin. Your tank must be at least 6 months old with lots of live rock as they eat the algae on the rock. Urchins are not easy to feed, I feed algae sheets, algae pellets etc and could only keep mine alive for the 7 months.
These beautiful creatures belong in the ocean or in a very large aquarium with lots of rock so they can graze naturally. I don't know if we really know what they eat in the ocean for sure but algae is surley one of those on their list.
With spines falling out, and possible attacks from other things in your tank you might end up loosing this urchin. Do yourself a big favor and save the headache by not getting another one, stars are something else I did not have luck with as 25 gal is just not big enough. I also just lost my Fuzzy Dwarf Lionfish. So here are three things I have kept in my 25 gal that just will not live in it.
I want to pass this onto you so you don't make the mistakes that I have. In my tank right now I have:
1 Coral Beauty Angelfish
1 White Striped Maroon Clownfish
1 Yellowtail Blue Damsel
1 Electric blue Hermit Crab
1 Large Turbo snail
1 Algae Eating Cowrie
I don't plan on adding anything else to this tank. I am maxed out but have sure learned alot so far.
I would be glad to help you if I can as we both have the same size tank. I am no expert but have been in the hobby for just about 3 years now.