Switched tanks and suddenly Ammonia!!

I switched from a 200 gallon to a 120 gallon so as to control the height out
of safety concerns of the glass shattering. All the water, fishes, LR/LS
from the original tank has gone into the new one.

However, upon testing I find that suddenly ammonia and nitrite readings have
occured. So far, .5 on both counts. Cant understand why that shld be so as
nothing new was added. Have restarted the Eheim wet/dry to control these but
am baffled and concerned that these may go out of control and kill all my
fish!

Any advice appreciated,

Thanks,
Ajay