When did you set up the tank? Is it newly setup? How long it has been running? The black spots sound like black algae. It does spreads around the tank on anything that isn't moving like slow growing plants. Algae grows if there is a chemical unbalance, like if there is too much phosphate due to lack of or too little water changes, too many fish for the size of the tank, etc. If it is a fairly new setup, the fish could have died for the same reason the algae appeared, e.g. high ammonia, high nitrite, high nitrate or too much phosphate. There are other reasons too.

We really need more info in order to be able to diagnose it.

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GUPPY STRAINS I'M BREEDING:

Blue Grass
Blue Grass Swallow
Metal Blue Grass Swallow
Red Grass
Red Grass Swallow
Neon Blue Giessen/Ribbon
Japan Blue Blue Grass
Japan Blue Wild
Neon Blue

Strains I developed in my fish room:

HB Yellow (own line)
Platinum Pink Blue
HB Blue Leopard
HB Leopard Multi
HB Red Crowntail