i used to have 25 gallon long and I used two 2-liter DIY CO2 bottles, which almost weren't enough. So I would use two bottles in your case and then it should be perfect.
my recipe (lasts 4 weeks):
2 cups sugar
1/4 teaspoon baking soda
1/8 teaspoon Champagne yeast
fill to the 'shoulder' of bottle with tepid/warm water
for best results, alternate changing the bottles so the levels of CO2 stay as stable as possible. You can also use baking yeast (Fleischman's) but lower the water a little because baking yeast foams at the top. Also, baking yeast will only last about two weeks before it needs to be changed. If your tapwater is really soft, increase the amount of baking soda to 1/2 teaspoon.
If you really don't want to use CO2, then use Excel instead. You should add 2mL per day for that size tank. But with 2.5 wpg, you're DEFINITELY going to need something for carbon/CO2.