For the 55 I'm thinking at least 3-4 2L bottles to be effective. Also, a warning about airstones... Co2 will breakdown the stone. I like using a piece of bamboo chopstick - the gas passes thru the bamboo fibers and the bubbles are very fine. If you want to splurge, a ceramic diffuser can be used with diy co2.
How quickly does the co2 break down an airstone?
How on earth do you use a bamboo chopstick -- Do you use the whole thing or cut it short? Do you jam it in the end of the tube?