Is it a metal tank? I used one of these last year in my garage and had a difficult time heating it. My garage which is insulated, built into the house, and is heated a little from the furnace and gas water heater. We did not open the garage door for about the first three weeks. I still had to use (2) 250 watt heat lamps for a few weeks. This year I am building a wood brooder. I am hoping I will be able to get by with one heat lamp. I will finish building it on Sat and then I will test it for a couple of days. I will be getting my chicks in a couple of weeks.
I suggest that you start with one and test it. Set it up with the bedding, paper towels, waterer, etc. - just like it will be when you have the chicks. Turn the heat lamp on for at least a few hours and watch the temperature. I ran this test last year and found I needed to add a second lamp. If you test without the bedding and other stuff you may not get accurate temperatures (in a metal tub the bottom can heat up quickly without a couple of inches of bedding).