Is .5 ammonia bad for the water in your fish tank?

Any ammonia is BAD. Your filter is not working well enough. To keep thing safe you will need to be doing some extra water changes (maybe daily) until the filter is working efficiently.

Ammonia should be kept at 0ppm at all time. It is toxic and leave permanent damage to the fish even if the fish survive it. Either your tank has not been cycled properly or the nitrogen cycle has crashed, or you have not cycled your tank at all. You need increase the partial water change to minimize the damage.