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I have seen these pods in alot of places, mostly in the bottom and in the back of the tank, they are about 1/2 an inch long and seem to have 2 main openings, i hope this pic is good enough.
BTW, the tunicates and sponges both are good to have. Not "pods" if they have openings. Looking again, they deffinately look like tunicates, have a few myself, and purple ones too.