The keepers of large aquariums are fimilar with what is called the Nitrogen Cycle.
It works like this:
You remove all the chemicals in your water supply so that it is safe for your fish. You put fish in your aquarium and feed them. The fish poop and the fish waists and uneaten fish food start to rot. The rotting organic waste gives off ammonia and in about a week the ammonia can reach toxic level because you don’t have any of the bugs that take in the ammonia and give off nitrites. Eventually those bugs will find their way to your aquarium. Some will float throught the air, some will hitch a ride on a net or come with the water from an established fish tank. Finding plenty of ammonia, they will multiply and make lots of nitrite and in about a week, but too much nitrite can also be lethal. So along come different bugs that consume the nitrites and produce nitrates. Nitrates are good (they feed the plants) But nitrates can also build up to lethal levels and so educated aquarium keeper do partial water changes to keep the level of Nitrates down to non-lethal levels.
You say, this is all nice, but what has it got to do with bettas?
Well for one thing, that is why you should change your betta water every week.
About the time you have free swimming fry, the ammonia level in that spawn tank is really high and by the time your fry are 2 weeks old, the nitrite levels are peaking. No wonder you’ve got so few fry surviving the first month.
What can we do about this ????
Start with water that has been in a fish tank for more that 2 weeks so that the bugs have already got the ammonia and nitrite levels under control. Avoid feeding the adult bettas and creating excessive food wastes / ammonia in the tank while they are spawning (and you have no filter). As soon as the fry are free swimming, remove the adult bettas and start a sponge (bio filter) bubbler on low. (Fry do not do well in strong currents.) Make sure you use a filter that came from another tank and already has a healthy growth of both kinds of bugs. Do not clean the filter and kill the bugs off. If you use a new or clean or even a dried out filter with no bugs, it will take 10 to 15 days for the ammonia and nitrite cycles to level out and during that time you will loose some of your fry. Do partial water changes, take out one quart of old water and add 2 quarts of new dechlorinated water until your spawn tank is full. Use an airline tube and if necessary, cover the end with a piece of old pantyhose to keep the fry out.