All of my fish died!

I moved from MA to FL about 2 1/2 weeks ago. The majority of the fish survived the drive in the car. I set up a hospital tank with a filter on a temporary basis until I could get their tank set back up. I purchased rocks from Walmart as I was new to the area and hadn't found a pet store at that point. I washed the rocks today & set up the tank. I put the fish in the tank. It was the same tap water that they have been in for the past 2/12 weeks. The only difference was the aquarium rocks. Within minutes a couple fish died. I figured stress from moving them. I left the tank light off and went out for a couple hours. When I came home I had 20 dead fish. I have 1 lone survivor who has been removed from that tank and placed back into the hospital tank. He is a golden gourami that I have had for about 3 years. All of the fish that died had been in the same community for at least 6 months. I am going to take a water sample from the tank with the problem and also the hospital tank to a store tomorrow. Has anyone ever heard of toxic rocks before??? Any thoughts??? I have had tanks for several years and haven't had problems, particularly with this tank for a long time. Of course I have never moved before either.... Thanks for any input.

I used a brand new carbon filter. Could be to little bacteria as everything was new. The strange thing is that the sole survivor is now in a hospital tank I had to refill (same water). I am running a mini filter for him with no carbon or filter medium because I had already tossed out the filter from when all fish were in hospital tank. He almost instantly unclenched his fins. The joys of moving....I can't find my test kit to test myself.

Definately does give me that opportunity. Right now I have a 55 gal with an Oscar. A 72 gal that is empty. And now a 30 gal that is empty. Never mind the 2 10 gal tanks that I use as hospital tanks. I think I might try salt water in the 72. It used to be my community tank. I don't know what I will do with the 30 gal. I can't wait to get the water tested before I do anything!!! Hope I don't move again for a while.

Aqua Culture Aquarium Gravel. It is sold in a 5 lb. bag. It is more of a medium size gravel. I actually have 3 bags left. I think I will take a rock sample with me too.