I don't think there's a "proper" way to dispose of it because quite honestly there's nothing dangerous (except sharp pieces that can come as a result). If you absolutely do not want to shatter it a few options... if you can fit the whole tank inside your garbage can, do that, cover with a tarp and drop a rock on it or something. (ok that still is shattering it
).
Other options, often your garbage company will have a bulk item collection they'll do, it varies by city of course, but it might be useful.
If you have the time, you might be able to use a dremel cut off all the plastic trim pieces and pull off the panes of glass and drop them one at a time in the garbage can. Use gloves though, the silicone will fight you to the bitter end!
But I know what you mean about the free stuff, I put stuff on craigslist and man people are picky as hell about free stuff. I wanted to give away some file cabinets, standard size, nothing wrong, people wanted pictures, wanted me to hold them, wanted me to call them back to see if they're still available... I finally put them on the curb with a sign that said "FREE" and anyone who sent me an email I replied with my address and told them if it's still on the curb you're in luck, if not tough s...ituation.