The issue isn't the open top. I didn't specify: I can put a lid on the second tank I mentioned. The issue is the size of the tank. I'm willing to upgrade to a bigger one eventually but my question is: roughly how long could I expect the tank I already have to be sufficiently large? Because if it'll only last me a month or two, I'll think about getting a bigger tank now. If it'll serve for half a year, then I'll use what I have for now.