mardarin in a 20gal?

[Ruth]

Oh please don't. Although there might be the exception a 20g is not going to give you enough pods to keep a Mandarin alive and healthy. Some will accept frozen or prepared food but IMO they still need pods in order to thrive. Without a large refugium where pods can multiply a 20 is just not enough. Do a google search or if you can search RC - there is plenty of information on that site.
I have 2 Mandarin fish in my 230g with a 77g fuge/sump and still worry that there is not enough for them to eat. Neither one of mine eat frozen or prepared food that I have ever observed.