our first fish loss

wrote:

Ammonia levels are fine, the aquarium guys agreed with me that the
water seems like it's fine. They also said that with 3 little goldfish
in 120 gallons with an oversized filter/pump and about a dozen plants,
it's unlikely to have problems with ammonia.

Tonight we introduced Sharkboy II, hopefully he will have a better
fate...

There's some misinformation there. First there is no such thing as a "fine"
level for Ammonia. If it's not zero, it's a problem.

Second, 120 gallons with a big filter (for a pond, it needs to be
appreciably larger than for an aquarium), three little goldfish, and
plants, shouldn't have an ammonia problem once it's established, but still
could while it's new. Especially if you have started with chloramine
treated water - some of the chemicals used to treat this will simply bind
with the chlorine part of the chloramine, leaving the ammonia to be handled
by your biofilter. If you have an established biofilter, you'll never
notice it, but in your case you may still end up with measurable ammonia.
--
derek