How long has your tank been up and running with fish in it?
How many gallons/litres of water does the tank hold?
How many fish were/are in the tank?
How many times a day and how much food were you feeding the fish?
There are a few possibilities here.
The water temperature could have fallen to a dangerously low level which in turn would have caused fish deaths.
Overfeeding--when you mentioned that "there appears to be enough floaties in the water for them to feed on" it's a sign of overfeeding as the general rule of thumb when feeding fish is to feed only an amount that the fish will be able to consume within 3 minutes.
Uneaten food in an aquarium begins to decay thus causing water problems.
You mentioned that water tests had been done but didn't mention exact results.
Ammonia should always test at ZERO. If any amount of ammonia was detected on the test kit it would indicate a major problem.
Nitrates should have tested 40ppm or lower although if they tested a bit higher would not indicate a dire emergency.
No mention was made if the water was tested for Nitrites but they like ammonia should also always test at ZERO. They too are lethal like ammonia.
An ph range of 7.0-8.0 (alkaline) is fine for goldfish.
In my experience I've found that when sudden and rapid fish death occurs it can almost always be traced to a water quality problem as a result of the presence of ammonia and/or nitrites in the water or an external toxin that has entered the water.
Best wishes and please let me know if you have any questions.
Actually a lot of foliage provides extra oxygen and consumes nitrates which are the end result of the breakdown of harmful ammonia and nitrites.
So I think that in order for the foliage to be a problem it would have to in some way impede the fish's ability to swim.