Sorry to hear about your fish. Other members are right about the daily water changes. Sometimes a fishkeeper can mess around with the tank too much and it causes harm to the fish. Daily water changes isn't a good idea IMO. A better idea is very heavy filtration, making sure the tank has been cycled a few weeks already, and picking up any leftover food.
These baby dats are very very sensitive and most members here have probably lost a few in recent months. Just be glad some of yours are still alive and doing well considering that you just got into dats recently. If the seller gave you lots of time to inspect the fish, there's nothing you can really do about the ones that died. It'd be a lot different if he shipped them to you sick.