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.


--
The G Man