Recently up graded my 120l tank to a 180l, when i did the change i left all the filter media in the water used all the same gravel (plus a little new) and used about half the original water then topped up with new. I also added a very mature fluval u3 internal pump so it had two oversized pumps onit. I let it sit for a couple of hours then moved the fish in. Over the next few days the ammonia kept goin up and up, i ammo locked it and didn't feed my fish for a few days but it got so bad i had to move some into my 4ft with the discus and the other gone on holiday to my lfs! why did this happen??? anyone know?