What could be wrong with my new aquarium?

A few weeks back I accidentally cracked my existing fish tank. It was only quite a small one so I decided over the weekend to treat our fish to a bigger and better tank. On Sunday I bought an Elite System 3 tank - it may not be a big aquarium but it's big enough for our 3 fish (19 Litres)

So, I proceeded to fill the tank, add the correct dosage of tap safe and started the pump. Yesterday morning I added some friendly bacteria (initial the dosage as stated on the bottle for new aquariums)

I then left the tank for another 12 hours at least before transferring the fish from the old tank to the new tank. Now, I know what most people will say at this stage (and maybe they are right) - I've not left the tank long enough to mature - but I've previously done full clean outs on the existing tank and the fish seemed happy enough in the crystal clear water.

The fish seemed fine when I went to bed last night - swimming around exploring. I gave them a bit of food which they ate so everything seemed normal.

I then got up this morning to find them all just lying still on the bottom of the fish tank - a sign which I've seen before to indicate things ain't right in there. Was a nightmare trying to get them out as they kept swimming into the new ornaments in the tank.

Have transferred them back to the old bowl - luckily I kept the old water just in case - and they seemed to recover immediately.

What have I done wrong?

Is there anything anyone can recommend I buy to test the environment in the new aquarium? I've seen a number of tests available but what should I be testing for?

Any help anyone can give would be very beneficial to me. Cheers.

Whoops... The website took so long posting my message that I thought it wasn't going to work so I posted it again. Please ignore this one. Mods - It you want, delete one of these.

What sort of fish do you have?

Geezer From The Freezer wrote:

What sort of fish do you have?

As posted in your duplicate thread, you have too small a tank for
3 goldfish!