If your fish are dying there is something else going on I believe. Aipastia wont kill a fish. I would keep doing water changes. Get some poly filter and put that in there to take care of the possibility of some chemical that may be in there. Also drop in some carbon just to be safe. The ammonia needs to be zero so water changes ought to help bring that down.

Have you made any changes lately?
What kind of sandbed do you have? depth?