From what I've been reading, open brains need feeding a few times a week, or they tend to die from attrition in 6 months to a year... I'll find the links in my history in a minute, but I'll share my experience first.
My open brain I've had for 2 months, has rejected anything other than finely chopped mysis. I tried larger pieces but they were regurgitated at night as was white shrimp and scallop (both finely chopped). My brain is 1.5 x 3 inch so size may make a difference... but consider that they are zooplankton feeders so they probably consume a lot of 'pod sized creatures. I haven't noticed any marked growth, but there has been no tissue recession and the color is as good as the day I bought it.
Now to find that article (if it was an article
).