Hmm the water change regimen is a little wonky. Hmm how shall I explain...
All fish secrete a byproduct called toxic substance ammonia/NH3 (aka Fish Pee). This is why we perform water changes: to prevent the ammonia from reaching lethal levels. There are two ways of managing ammonia:
1) UNCYCLED TANK: Frequent 100% changes are carried out to ensure that the ammonia is reasonably close to 0ppm. For this method of ammonia management you will need an ammonia test kit so that you can monitor ammonia levels and make sure they are nice and low.
2) CYCLED TANK: You let the nitrogen cycle take care of your ammonia for you. When ammonia is allowed to build up in the water, a type of nitrifying bacteria start breeding in the water. They eat the ammonia and what comes out the other end is a less-harmful oxide of ammonia called nitrIte (NO2-). These bacteria live predominantly in the filter, with a good chunk of them also being found in the gravel.
When you get enough nitrite floating around the tank (by now your ammonia should be zero since all the bacteria are hogging it), a second type of bacteria starts proliferating. They eat the nitrIte and what comes out the other end is a compound called nitrAte (NO3-). Nitrate is only harmful in high amounts. The nitrate is then removed through weekly water changes of about 25%, via a gravel vacuum or siphon hose. Plants can also use the nitrate as fertilizer.
So since you have not chosen whether to leave your tank cycled or uncycled, what is happening is that the ammonia level is gradually reaching a peak, ten just before bacteria can kick in, you rinse the gravel and away they go. If you go with one of the two you should find that the tank remains odorless.
Either way, I strongly recommend a test kit. Stay away from strips or indicator dials; they are quite inaccurate.For uncycled, grab an ammonia test kit and a pH test kit. If you go with cycled, go whole hog and pick up kits for ammonia, pH, nitrite and nitrate. API makes a good one that tests all four of those-- just picked one up for myself! Good price, too.
You can get more info by looking aroundthe site or googling 'aquarium cycle'.