Here is what my research today yielded...I'll post not knowing whether or not you found what you were looking for yet. I didn't see the black fluorite yet, however I saw eco complete. Since your profile says you live in Minneapolis, I'm going to guess Richfield isn't too far off?
Aqualand - on Chicago Ave in Richfield...they had 1 bag of eco complete. Priced at $29.99
Something Fishy - on 66th in Richfield...they have several bags of eco complete, $24.99
World Of Fish - on 66th in Richfield...they had a few bags...but eep...$37.99.
Forest Lake Pets had lots of bags too, but they are a bit north of the cities, up 35, a little bit past where E and W join again. $24.99.
Hope this helps, I'm not sure how far Richfield is from you.