Could someone identify this please?

[justinl]

If you're considering biological control (which is always my first choice... Im wary of the chemicals), think about what is appropriate for your tank. For example, although a CBB can be a good choice, it really isn't appropriate in a 55g. In fact that tang you lost should never have been in a 55g either. Don't mean to chastise you and act high and mighty or anything, just letting you know.