bit surprised my shop said 25% fortnightly was standard.
You'll find that
most pet store employees will tell you what they think you want to hear, which will hopefully make them a sale.
Every fish that dies in your tank that you replace from his store, every disease you buy medicine to treat, is money in his pocket.
I once heard a Petbarn employee tell someone that the nitrogen cycle was a myth, just so they could sell a few fish that day. Unless you have a really good LFS take everything they say with a grain of salt. Especially chain stores, that's where most people get bad advice.
Anyway, you've been given some good advice from other members.. Clean water and good food make for happy healthy fishies.