What happened to my aquarium fishes?

ok i brought some natural plant for my 20 gallon fish tank because the plant was looking quite nice and i was looking some thing to decorate my tank so i brought the plant but my glodfish pluck out all the plant from the roots and all was floating on the top the next day i again fixed it back into the sand and again the next day all the plants were floating on the top so i decided to leave it like that but after 3 4 days all my fishes stopped eating and lay at the bottom of the tank and breating sloly than normal and then next day 2 of my fishes died and another day one fish died and after 2 days all the fiah died i had 7 fiahes they were about 2 years with me and now non of them are left i am feeling very sad but what could have killed them i treated them by putting 4 teaspon of rock salt and one yellow which i used to put to prevent lick and fungal infection but nothing happened did they died due to decaying of the leaves the plants were aquatic and were in an aquarium with some fish and snails and snails babay and i also found some jelly like thing in the bottom of my aquarium when i was cleaning after all the fish died it s colour is like of water could it had killed the fish i don,t know what was it but i found 10 to 12 in my tank it was round in shape