Not sure of the dimensions of a 55 gallon tub, so I'd go by measurements for whatever things you're considering using as a brooder. I think for the first week or two, it's 6 sq. inches (minimum) per chick, then up to a foot per chick, and by a month or so it's 2 sq ft. per chick. Just like with grown birds, more space is always better IMO. I don't know what your coop set-up is, but if it's draft free, you can always use a brooder lamp out there to get them into more space (and out of the house) sooner. Mine just spent their first night out in the coop, and they're 2 and 3 weeks old; they had a lamp of course, as we got down to 40 last night.
20 is a LOT of birds to try to keep indoors for more than a few weeks...
Oh - and I'd be sure to do some kind of lid on whatever you use. They can and will fly around by the time they're 3 weeks...
Nice brooder abercrombie!