I used Prime for many years before switching to powdered dechlor/amine (Chloram-X and more recently Safe or powdered Prime).
Adding several caps of Prime to a 50g bucket is many times more than needed...but adding water from a bucket with ~6-7 times the dose of Prime to 10% of the tank shouldn't cause a problem.
Seachem says that you can use up to a 5X dose (see below).
Use 1 capful (5 mL) for each 200 L (50 gallons*) of new water. This removes approximately 1 mg/L ammonia, 4 mg/L chloramine, or 5 mg/L chlorine. For smaller doses, please note each cap thread is approx. 1 mL. May be added to aquarium directly, but better if added to new water first. If adding directly to aquarium, base dose on aquarium volume. Sulfur odor is normal. For exceptionally high chloramine concentrations, a double dose may be used safely. To detoxify nitrite in an emergency, up to 5 times normal dose may be used. If temperature is > 30 °C (86 °F) and chlorine or ammonia levels are low, use a half dose.
I've been using these products for years and - I'm sure - routinely overdosing by not measuring / just adding a pinch to each tank before re-filling. I'm also sure that I routinely add an extra pinch to tanks that I've forgotten whether I've added dechlor or not... I don't have chloramine (only chlorine) but I can't recall a situation in which I've had a problem (and certainly not a mass die-off) as a result of over-dosing Prime (Safe or Choram-X).
Seems like there was something in the tap water that day that usually wasn't there. Could you call your water authority and ask if they were doing something different that day (and whether there's some place that they post that kind of info)?
Matt
