Firstly, GREAT TANK!!
Secondly, deals like that are not uncommon at pet supply stores, especially if we (I work at one) are going to stop carrying that product or we have a reset on inventory. All pet stores do it, especially the larger chains. I got a rat cage that was normally $100 for $20 because of a product reset and there was no room for that type anymore (despite it being a VERY popular brand). You just gotta go into the store every week and look around. At my store, we receive supplies on Tuesdays. I tell my regulars or bargain-hunters to come in on late Wednesday or Thursday for good deals.
The trick is finding out when "truck day" or "supply day" is for your local store, and paying attention to when sales are ending. When a huge sale comes to an end, left over stock is always clearanced out for the new sale.