Another dead one this evening.

Also have noted that one of the fish is smaller than the others and trying to steer clear of the others and they seem to be hassling him a bit. I fear that this one is next, I don't have another tank to move him too. I do have a small tank in the garage but this is not cycled.
Water readings
Ammonia 0
Nitrite 0
Nitrate 20
GH - 18 drops/322.2
I am just about to do a water change, not sure if there is anything else I should be doing. Should I be trying some meds?