By no means am I a pro but I would start with a heavy grit sand paper like 100 or 80. then work your up to your desired feel. I would probably at stop 200 or 220 but im a glutton for punishment. Then stain and satin or semi gloss poly. Let the poly dry and do a very light sanding with a very light course paper or wool. MAKE SURE POLY IS 100% DRY or you will see the steel wool. Repeat as necessary. I would do at least twice.
Thanks! I was not clear in my original post. I know how to finish but, need to do it while my reef aquarium is in use. How do I do it and protect the tank inhabitants?