Clean water is the most important thing you can do for developing fry. I've found that I have to do more water changes with live brine as opposed to a dried food. If there's leftover brine that dies, it tends to rot pretty fast because of its small size and fouls up the water. I raise out fry in bare-bottom 20 gallons with a heater and sponge filter. They get 50 - 75% water changes every day depending on the number of fry and the amount I'm feeding. If you're worried about shocking them with new water, set up a second tank or a rubbermaid container to age the water in overnight. Then you can match the temp and parameters before the water change and it won't shock the fry.
I rarely ever feed brine now. If I do, I get the Hikari frozen baby brine in small cubes. Most fry will eat this pretty well. You can also try Hikari First Bites powdered food for really tiny fry. I take a pinch of it, mix it in a small cup of aquarium water, and blow it directly into the swarm of fry with rigid airline tubing. Once they get big enough to take larger particles of food I've now started using New Life Spectrum growth formula. It's basically a crushed/crumbled pellet. I've got a tank of Heros sp Atabapo fry and a tank of Krobia guianensis fry growing out solely on that right now and they're doing great. They get water changes @ 50% every 1 to 2 days.
There are other factors that contribute to fry deaths. Gill flukes will start to drop them like flies around the 3 - 4 week mark sometimes. It could be a bacterial issue. It's really hard to say for sure. I don't feel that this is as common as water quality problems, so I would always start with the water and see if that fixes it.