Did you accidently kill all the old bacteria in the old filter (left pump off
too long, let dry out, ...)?

Old filter is working perfectly. Readings in last chamber show zero
ammonia and nitrite, water is then pumped into sand filter and through
to pond where reading is high ammonia. If I bypass the sand filter the
ammonia level in the pond is back to zero in 2 or 3 days.

Maybe I am getting dead spots in the sand ? I will reduce the sand and
try again.

Carl