All parasites have a life cycle some more complex than others, some have direct life cycles meaning they need a host to survive, then theres indirect which means they can live with or without, but without a host their life cycle is very short about 2 weeks, if it were me and the tank was fish less I would increase the heat to 85-86 do water changes every other day with thorough gravel vacs and reintroduce fish after a couple weeks, (before you add fish lower tank temp Back down to 72-78), you could also just dose the empty tank with prazipro or general cure if you think it was internal parasites, but just out of curiosity what was going on with the fish before they died it might not be parasites, did they look like they were covered in salt/sugar (ich) ? What were the symptoms of the fish?
This isn't a silly question though it may come out like one, when you fed them did they eat? Or did they try to eat and spit it back out or no interest at all
