hi i recieved my fish yesterrday at about 10.30am. i floated them for 30mins as advised and all seemed well. at around 10pm noticed problems and sum dead fish did water change and double checked everything. lost all ten neon tetra but my 2 plattys and 2 black swords seem fine. really upset me has this and i dont wanna risk it happening again buyin more fish but only got four fish now can anyone explain why this happened