Effect of too many fish

Michi Henning wrote:

I suspect that 40ppm or more wouldn't be unusual in many fish shops.

Why would you think that? Every time fish are sold from a tank,
some of the water is sold with them and replenished at the end of
the day. Except for tanks that hold low volume sellers, there is
probably about 25% water change in these tanks on a weekly basis.

Also the shops are more likely to underfeed than overfeed their stock.

What sort of conditions would be necessary to get ammonia up to 40ppm?