I kept a 40 gallon tank for years, and I pretty much stuck with small yet hardy fish - like fancy guppies or neons. I liked black mollies, but they are so prone to ick that I gave up on them. I always had pretty good luck with angel fish, silver dollars, and red sword tails.

I can't imagine any site saying a full tank cleaning every 5-7 days tho, unless they might be referring to betas in a bowl with no filtration system. I can't imaging a 1/3 water exchange every week either, even with a tank with a reasonably good filtration system. The less the water exchange and the less often, the less shock to the fish. Most of the time I did more than 1/3, but I NEVER did more than 1/2, and definitely not every week either. Of course the more fish in the tank, then the more often it needs to be cleaned too, but certainly not every week. Not in this house anyway!