I would normally do 50% waterchanges weekly as a normal thing. Some people spaz when ya mention more than 30% and will advise ya do 50% in 2 different changes per week... really either is fine.
I would do 100% worth of waterchanges per week when there are torn fins or something of that nature, but NEVER a 100% change at one time... either (2) 50% changes or (4) 25% changes or whatever, per week.
Yeah, that 25% per month or two is very outdated info...
If you are on a well for your water you wouldn't have chlorine and would not have to add anything to it, but if you are on a city (municipal) water supply you will need to add dechlorinator to the water. Either way, the water that goes back in the tank needs to be within 2-3 degrees farenheit of the tank water already in the tank to avoid putting your fish in shock.

I hope that helps.
