All the fish in my fish tank have died except the 43 babies?

i have a fish tank that i recieved on february 12th. I had a sunset molly, three gost shrimp, a frog, and a crab. the crab killed the frog. the fish turned out to be pregnant and gave birth to 44 babies. one died. the mother dies 4 days later after she stomach appeared sucked in and mis-shaped. the babies remained in a seperate tank. A shrimp then died, anothers brain turned black and dies 2 days later. the last one is missing. The crab dies the day after the shrimp and his stomach was coming off of him. The babies remain fine and unharmed. There were high levels of nitrate and nitrite. I've treated the tank for two days. I got a sucker fish to take care of the allgee and it died the day after i got it (and it ate all the allgee in the tank) The babies are still alive and well. Can anyone help me???!!!! PETCO and Petsmart havn't been able to help.