Which of your tanks are you having this problem?
If you have not allowed the tank to run fishless for at least three continuous weeks, now would be a good time to do so. This will allow and fish born parasites (ICH) to die off from lack of a host fish.
I had similiar problems in my first 4 months or so on my 55g. Fish would last three weeks, start breathing hard and then die in a couple of days. In the process nitrItes would bump up then fall after not feeding for a couple of days. After additional running time the nitrites would not bump up and fish would live. Just had to build up the bacteria for the nitrogen cycle.
I also had low ph which killed some fish in days instead of weeks. again additionaly running of the system build up buffering and now ph stays at 8.2-8.4.
If you do not have macro algae or marine plants in the system, then I would add some. They filter out all types of bad stuff, remove carbon dioxide which allows more oxygen in the system, buffer ph, and generally balance out and stabilize the system.
Sorry about your losses, but hang in there and take it slow. We have all been there.