The question about whether the tank is fully cycled is a good one.
I think your problem may be a failure to properly drip acclimate the rummies before adding them to your tank. Did you drip acclimate your fish before you added them to the tank? You want to give new livestock time to slowly get used to your tank water before you add them to the tank. Trying to match your tank parameters to the tank in the LFS is not the way to go, the fish will be living in your tank not the one at the LFS.
When you bring home new fish take some of the excess water out of the bag and float it in your tank. I use a small clip to attach the bag to the frame so it doesn't sink and fill with tank water. Add small amounts of tank water to the bag every ten minutes or so, the longer you take and the slower you go the better. I never take less than an hour to do this and sometimes drip acclimate the more sensitive species for up to three hours or more depending on how large the pH mismatch is to start. Over time the water in the bag will equalize and be a much closer match to your tank water, the fish will also have time to adjust to any differences in water chemistry. For the most sensitive species check the pH in the bag to make sure it is the same as the tank before moving the fish into the tank. Sensitive species of fish like rummies and cardinals stand a much better chance of surviving the move if they have been drip acclimated.