Seachem do advise to refrigerate if it will be open for more than a few weeks (I've forgotten the exact time they suggest). I keep it in the wine fridge which is not as cold as the fridge but still cool. Before i did this (I asked Seachem about it at the time) I did notice that what is normally (initially) a dark brown liquid became black by the time I was near the end. I do shake it before every use so it was/is not a matter of it necessarily breaking down. Refrigerating does seem to prevent this, so there may be something to it. I buy the smallest bottle now, as with my downsizing I have fewer tanks, and this lasts for months.
I freeze dry prepared foods (flake and sinking) if I buy the larger size (more economical) and just take some out periodically. Someone on here suggested this, AbbeysDad I believe. Once things are exposed to the air they can degenerate.