Fish appear to be dying

Question
Hello, we recently got a 30 gallon aquarium and set it up.  We set up the tank and had the pH levels and stuff checked by the people at the pet store before we introduced any fish into the tank.  The water temp in the tank is between 76 and 78.  We had 3 angel fish (one has died) it just became pretty sluggish seemed almost paralyzed was breathing slowly and then died.  Now the plocostumus is doing the same thing.  A few of the Guppies are also darting wildly around the tank.  We also have a albino Catfish in the tank that occasionally does the same thing.  The 7 tetra's seem to be fine as do most of the other fish.  The water in the tank is also kind of cloudy.  All the plants and rocks in the tank are plastic and had a hot wash before we put them in.  Please Help I really don't want to loose all my new fish.

Answer
Hi Shannon,
It sounds like your tank isnt fully cycled. The classic symptoms of high ammonia levels are darting as you described. When you get your water tested make sure you ask them what the exact levels are. Some will tell you they are "fine" because they want you to buy fish from their store. To cycle your tank do daily water changes of about 30%. Ammonia and nitrite should be at 0 and nitrate should be under 20ppm.

Hope this helps
Chloe.