sorry tank info.
milennium 3000 filter
Biological filter was sufficient for the one african you had before adding two more.Sudden addition of more waste for filter to process, and perhaps more foods offered,could have resulted in the death of fishes.(ammonia and or nitrite spike)
Is also possible that one of the new fish was sick and as Lupin mentioned, without quarantine,,the possibilty that one of the new fish brought pathogen unknown to your tank is real possibility.
Is literally no hope for longterm health of fishes with water change every six months.(unless tank is heavily planted and sparsely stocked with fish)
Weekly 25 to 50 percent water changes and gravel vaccum will help maintain the health of fishes and remove those things like the worms you see.
If your tank is 28 gal,, then you could not hope to keep even one Oscar in such a tank for more than a couple months.
The Oscar's are large fish as Adults and need large tanks that will allow them to grow properly and they produce much more waste than four or five africans.
Excess waste = more water changes assuming you want the fish to thrive, and upgrades in filtration will be needed as the fish grow to possibly twelve inches inside a year with proper care,hence the minimum 75 gal to support just one of these personable fish.