Water Changes - are a thing of the past for a while. You will need to resort to a very gentle syphon of the muck on the bottom of the tank for several weeks being extremely careful not to suck the fry up, use a very very small tube. Add a very small amount of warm, treated water to replace what you remove.
If you haven't already check out these sites - I find them really good
www.bcbetta.com & www.bettatalk. not sure what the rest of this is.
You will need to work out the best method that suits you but raising fighters, while it is both time and space consuming can be very rewarding.