Most store do not take old tanks for trade-in. You might have better luck trying to sell them on the 2手硬體交易區
If no one wants your old tank, you can call your local city trash collector (清潔隊) to schedule a time and place for pick up (free service). You need to take them to the designated location yourself.
I didn't think store would but, I thought I'd ask just in case. The tank and stand are less than a year old and in great shape. I'll probably keep it.