Why do our guppies keep dying?

we have a jewel delta 300 that has been set up for about 2 years we do regular water changes and checks(all levels seem fine?) before the weekend the tanks stock was : 1 rummy nosed tetra,4 neon tetra, 3 clown loach,2 leopard corydora,4 albino corydora,1 albino chinese algae eater,1 algae eater 2 danios. we wanted some 'top feeders' so purchased 4 male guppies and 3 more rummy nosed tetra and a wood shrimp, within 2 days 2 guppies disappeared, the next day the shrimp was dead and today another guppy is missing( i would just like to point out we have had this problem in the past when we purchased guppies and we also seem to have the same problem with gourami!!!) the rummy nosed tetra we bought are fine!!! we can not figure out if the guppies are dying and then being eaten or just being eaten???? please help??