2 dead stingrays

2 of my stingrays died out of 4. it was the pearl and flower. about 7 inches in radius. i noticed only the stingrays with white rotting tails died [ i just came back from a short holiday trip to found they had rotting tails] so i thought it was the suckerfish that bit their tail so i moved it. 2 days later, the flower died, 2 days later the pearl died. but the motoro and polkadots while DON'T have rotting tails seem fine so is it because of the rotting tails thats y they died? we changed the water quite frequently and avoid overfeeding. pearl seemed so fine yesterday, he was swimming and eating and i woke up to find him dead!
i didn't think the white rotting problems. i thought it was gonna heal but maybe it was an infection and was what lead to the death??

my holiday was only 3 days long. told my mom not to tell the maid to feed the fish and she still did and i suspect it was because the maid doesn't know the right amount to feed it. the water looks clean but there was a lot of shrimp shells everywhere.
so one of the reasons could possibly because of overfeeding or infection ;P