Why have my tropical fish died?

We have a new tank and have dechlorinated the water and added bacteria and plants. The temperature is 25 degrees and we have an electric filter, an airstone and a light. We added two guppies and 12 neon tetras. All were fine until the next day when the tetras were found lodged in plants, ornaments, gravel etc. Most were dead. The few that were left alive were listless and bobbing around the tank carried by the currents - only capable of twitching. Over the course of today the two guppies have sunk to the bottom of the tank, becoming listless. They are now lying on their sides and occasionally turning upside down. Can anyone suggest why this has happened and how can I prevent it from happrning again?