I recently got my bettas to spawn, but this is my first time. I have done a lot of research and it says I can't change the watt until the fry is about 3 weeks old. The problem I'm facing now is that the water in my tank is evaporating. I fear that by the time I can change their water it will be too late.

I read that to prevent that I would need to cover the top some how. At the moment I have nothing to cover it with, so here is my question. Can I condition a cup of water everyday and add it to the tank? Will that kill off some or all of them? If should I wait like a week before I start doing this? anybody can help me, please I really need to know.