You might get more interest if you gave more details on what you're selling.
Just the tank or the tank and the custom built stand? Are you including any hard scape, plants, fish or equipment in the sale? Is it available immediately or will you need time to move out the current fish and equipment? What's it look like now and--given it's a homebuilt tank-why are you choosing to sell it?
Lastly, give a price or price range that you're hoping to get or compare it to another tank with a known price. You can't expect others to read your mind as to what constitutes a "lowball" offer so there might be folks interested who aren't responding 'cause they don't want to deal with you getting in a huff over a too-low offer.