Hello and Welcome to the wonderful world of becoming a betta mommy, It would depend on how often you are changing the water, and what size tank you have your lil fishy's in..if the water is stagnant, you may encounter lil bugs growing in the water, eew not a good thing the water needs to be changed more often, and oxygenated for air to circulate and it won't stagnate.
When I first got our betta, this is what happened before I upgraded a basic Petco bowl for a 3 gal tank with an aeration stone, for the oxygenation, I was quite alarmed and realized how much water needed changing, especially in smaller containers, also they are jumpers and if you don't have a lid, your fishy's could jump out. just some things to check out,
being a betta mommy does take work, and it's good that you found this forum to get all the answers your looking for. I hope this helps, look forward to seeing some pix of your lil guys:)