yes, you should be topping off with freshwater, and doing that daily. If you find that a problem, then i suggest an auto topoff unit, tho imo
they're not needed. Some will say "well i need one because i'm always at work and dont have time to top it off..."
So when do they feed the fish? How bout water changes? imo
topoffs are over rated and i see no reason why you cant just top off with a jug of water whenever needed. Now i'm not bashin on you in any way at all, so dont take it like that. I'm just saying that people in general make topoffs a bigger deal than they should, so just dont let that get to you..
enough rant tho lol..basically just make sure that you top off whenever you see that it's low, which for me is once a day, sometimes 2.
Do you have a glass lid on your tank by any chance? Just wondering cuz you say that you've never had to top off before....