Best way to keep a mandarin to to have them eat frozen food or pellets. I purchased an ORA mandarin, they are supposed to eat pellets. Mine did not, but after a month of intense feeding several times a day by target feeding my mandarin took to some frozen. It took a few more years but mine now eats pellets. I must have tried over a dozen different frozen foods though and my water parameters were just plain terrible due to all of the overfeeding.
Even now, years later i still overfeed, cause it is possible for a mandarin to go back to pods
only at any time. This is undoubtably the hardest fish I have to keep. Not sure i would do it over again if given the chance.
In that size tank you would need more than 1.5 lbs per gallon in my opinion as well as have established rock. ALot of established rock. You want to see pods
everywhere in your tank, by the thousands. Thats the only way my mandarin lived for the month it took to train her, she wiped out every last pod I had. i even had a pod tank at one point. lol