What happened?

I was visiting my friend today who has 7(!) goldfish in a 40 gallon
tank. He lives beside the river and I collected some plants and snails
for my river tank. We put one piece of elodea in his tank for the fish
to eat and 2 largish snails.

He had bought some plants 3-4 days ago and I took them out of their
pots and buried them in the sand. Java fern tied on driftwood and 2
Amazon swords.

A couple of hours later he phoned me to say that his goldfish were all
lying on the bottom of the tank looking stunned. The pleco was fine.
He wanted to know what I had done to his tank!!!My friend immediately
did a partial water change and the fish started feeling better.


The only thing I can think of is disturbing some toxic gas pockets in
the substrate, which I thought I had just read was an old wive's tale.
He has about 1 inch of beach sand on the bottom. The goldfish are
always rooting around in the sand. My friend does regular testing and
water changes. About 3-4 days ago he put an extra pad in his filter
for phosphates due to a problem with green water.

Any ideas?

Thanks,
Judy