Something's wrong...

Something's wrong with my fish tank... 6 tetras have died over 3 days... had quite a good streak before that... think ultimately, the best way to keep the fishes alive is to cause minimal change to the tank... prior to that... when i increase the food supply, some die, then i decrease, then some die, then i put in some chemicals to improve the quality of water, somehow the next day, i saw clumps of mould like things appearing all over the tank on top of the sand... and more die... so I decided i'll do a major change... i removed all the sand, removed the plant, cleaned up the tank... after that 1 of the sick looking fish died, but on the good side, the remaining 5 looks ok... hopefully they'll remain well this way...