300! Congratulations! Here is some info on Betta fry care. It's more about jars.
http://www.thebettabubble.com/pgs/care/cares.php?id=4 Not sure what you have on hand for a filter, but here is one recommendation at
http://www.aquariumhobbyist.com/articles/BreedingBettas.html :
"Filtration is something that we don’t normally think about when it comes to bettas, but since they will have a couple of hundred fry it is a must, however, don’t run out and buy an expensive power filter that will suck up the fry. A good filter to use is the old-fashioned box filter that sticks to the corner of the tank powered by an air pump. There are several types of media that you can use in these filters, but since you will be using a mild medication to prevent fungus, the carbon the filter comes with is not one of them. Instead fill the filter with floss or sponges."
If what you have has an intake like a hang on the back filter, you can cover it with a piece of nylon or other similar material to keep the fry from being sucked in. Other than that, the filter should be low power so the output does not create a lot of turbulence. That's why the old fashion type is recommended above.