I guess if you don't want to spend $50 we're not talking pressurised CO2.
Don't bother buying one of the off the shelf CO2 system. They are all pretty much a bottle with some yeast, sugar, maybe some bicarbonate of soda, some tubing and a CO2 reactor / ladder. Very easy to do yourself.
You need:
A 2 litre coke bottle
Yeast (wine yeast is better, but bread yeast is fine)
sugar
airline tubing
A CO2 reactor (some make them, but I bought mine. Hagen do a CO2 ladder which works and is cheap).
non-return valve (cheap one you use for airline works, just put it in the airline to the CO2 reactor to stop water syphoning back into your coke bottle from the tank.
Drill a hole in the top of the coke bottle top smaller than the diameter of your tube and pull it through with pliers, or use silicon sealant to make a seal.
Then... to get you started.. Add 2 cups of sugar, 1/4 tea spoon of yeast and 1/4 tea spoon of bi-carb of soda to your coke bottle. Fill with luke warm water to within 3 inches of the top, secure the top and plug into the airline going to the CO2 reactor. 24 hours later you should have CO2 bubbling up your CO2 ladder.
For a 55 gallon tank, you may well want 2 * 2 litre bottles.
At night, you may want to run an airstone to help keep the CO2 levels down, so you don't stress the fish.
Lots of information available under DIY CO2.
Cheers
