Since you are losing that much water over a week, just do your routine weekly water change and add back a bit more than you took out. Never simply add water to a low tank. It seems so innocent to do that but what you are doing is concentrating any mineral content and contaminants in the water. What evaporation does is remove water only. Any and all dissolved minerals and contaminants are lefty behind. If you just add more water, the new water from your faucet contains all of those minerals and over time the concentration will increase. Instead you can do what I do for a top off and add purified water but, for most people, that becomes too expensive to do. The other alternative is to do a routine water change. You remove water that has minerals dissolved in it and replace it. Since any concentration increase that might have resulted from the evaporation is being partially removed and replaced, the strategy minimizes the increase in mineral content, but does not completely prevent it. I use an RO for drinking water in my home so purified water is available for me to use at minimal cost. If I bought enough at a store to do a top off, I would be better off simply doing a water change in terms of costs. As it is, m,y own tanks get routine water changes but top off water either comes from my RO or from rain water. Believe it or not, rain water, at least where I live, is very pure water. It is lower in minerals than the best water I can get from my RO by a factor of about 2.
if your losing a lot of water then its fine to just top it up when needed 
