Just like us, your fish need food, water and oxygen to survive. However, there's one a huge difference (besides having opposable thumbs!) - we leave the toilet behind when we flush and walk out the bathroom door. Unfortunately for our finned friends a fish tank is very similar to a giant toilet - especially when you don't perform frequent water changes on it. The smaller their tank is, the more truth this holds because there is less water to dilute the toxins from their waste.
If you were to follow the advice from most of the large chain pet stores your fish would still be swimming around in their own sewage. "But I always do a monthly 20% water change," you say! Sadly, the recommended frequency, and quantity, that is touted by most pet store employees is completely off. By only taking out 10-25% of the water in your fish tank set up once a month, you are barely skimming the surface when it comes to all the toxic waste floating around in there. Think about it - your fish have had an entire month to eat and poop before you even pull out the siphon. No wonder so many people have fish problems!
The more water changes you do, the better the water quality in your fish tank - and subsequently, the healthier your fish will be. As long as you are performing them on a regular, frequent basis - and always using a quality tap water conditioner - your fish will thank you for the breath of fresh air. Large water changes will not shock your fish if they are frequent and regular, because at a frequent interval there isn't enough time for the water chemistry to radically change. Just make sure that the temperature is roughly the same, because a fluctuation in water temp will put them into shock.
Contrary to what the pet store employee may have told you, a water change larger than 20% will not kill off your bacteria colony. How could it? They don't colonize in the water column. Your bacteria is found on hard, porous surfaces - like the gravel and the filter pad. So if you're going to do a water change, don't wipe down the glass, change the filter pad, and extensively clean the gravel and/or decorations all in the same day.
Frequent, large water changes are one of the best things you can do for the health of your fish - it will also promote better color and faster growth. After all, a sick fish living in poor conditions isn't going to grow very quickly, and it certainly isn't going to show you its best colors! Find a water change schedule and routine that works for you, and stick with it. I personally like doing mine twice a week, around 50-70% of the water at a time.