It blocks the filter current so that your betta isn't blown around like a leaf. Their huge fins catch currents like a sail. The easiest method is to make one out of a water bottle. You should be able to get instructions off google or even a step-by-step video on youtube. I believe there are also a few topics on it here.
On the cleaning note, it highly depends on your tank size... the solution to pollution is dilution! So the less fish per the volume of water, the more dilute and harmless your waste will be.
Cleaning the tank out completely is a good route to go if your tank is 'uncycled', eg deviod of a nitrifying bacterial colony.
In cycled tanks, the nitrogen cycle is put into effect. Essentially, you cultivate 2 kinds of bacteria which feed off of fish waste. The first kind eats toxin produced by fish waste called ammonia, and what comes out on the other end is a less harmful compound called nitrites. The other guys, whose favorite snack happens to be nitrites, snarf the stuff and produce a far less harmful compound called nitrates. These can be used by plants, if you plan to go that route, or simply vaccumed out at each water change. You will for obvious reasons want to limit your water changes to 20 or 30% because
a) You don't have to do full changes
b) You dont want to make too big of a dent in the bacterial colony.
Hope that helps!
Last edited by kelly528; 10-22-2009 at 12:26 PM.