Are you doing water changes? What kind of filtration do you use? Is there any detritus in parts of the tank?
I'd stay away from liquid supplements, in my experience they can be hazardous. First, I've been told they can 'saturate', and re-release the toxins, doubling the problem. Second, you probably want to figure out what the problem is, rather than using a quick fix.
If there's detritus (waste built up), hermit crabs will help remove it and stir up stuff so your tank can filter it out. Also, it's controversial to say this, and please read on it before taking my advice, but you may also try adding some macro-algae to your tank, rather than building a refugium. As long as you don't let it get out of control, in small systems it can be aesthically pleasing and functional. I keep feather caulerpa (very small amounts, it grows like bermuda grass), letting it physically use nitrates to build itself up, which I remove when I prune it. Read up on that, but it may be an option. Let's see if I get reamed for mentioning it.