I think a bottle of pods a day is just a littleee over exaggerating. Mandy's in 20 gals can definitely be done. I really don't think its as hard as people say it is either. I've seem them kept in 10's successfully. Although, you just added him too soon. Take him back to the store, and have them hold him for you for a month or 2. In the mean time, seed your tank with a few bottles of pods, and allow them to multiply. Then, have a spot in your tank that the mandy can't get to. I have a little shower caddy deal stuffed with chaeto inside that I zip-tied with window screen to close out the top. Or a pile of live rock rubble will do. When you add the pods, inject them with a turkey baster or whatever into the rock pile, or whatever you end up doing. Add the little guy after a month or 2 and you'll be good. Then probably add another bottle of pods every month or so. If the mandarins belly starts to sink in, you'll know he's not doing well. If he's getting fat, your doing good. BUT. As is, everyone's right. It'll die.