[Delphinus]

Well ... hmmm. You should still be able to see through the tank lengthwise.

This is what I'd do. First, make sure all inhabitants are accounted for. Next, run some carbon (I like to use a hang-on aquaclear with a carbon bag stuffed in it, but you can dump it in your sump if you can get it so the water flows through it ... whatever works). Maybe even run some polyfilter. You know what's really keen? You know those quick-filters you can get for the Hagen powerheads? Well, the filters you can get for the power-filters, one of those sizes (I think it's "No. 4") fits a quick filter perfectly. And it's a combination polyfilter with carbon. Just put this in the quickfilter instead of the normal quickfilter pad, and jam it on a powerhead, and you're set.

And a water change couldn't hurt.

Have you tested your params? Anything amiss?