Cow0bunga

I live in the US and am a Mom of three.. My youngest daughter has wanted a fish tank for a very long time. We fish sat for some friends with a 19 gallon tank for several months. When I started looking for a fish tank I may have overdone it a bit. We bought a tank online with filter light etc.. We sat the tank up and left it for three or four days. The lady had given us the rocks witch she said would contain the good bacteria we needed... We then went to pet store and picked and out ( with the help of the sales person ) compatible fish. We purchased about 15 fish in all. Mostly guppies, mollies, a tiger catfish, a bubble fish, and 6 glow fish . We had two frogs and an bristle nose pleco the lady gave us when we purchased the tank. This is where the nightmare began. The bubble fish was dead before we got it home... The man at the pet store out all of the fish in the same bag. By the next day we had list two more... I explained to my daughter that this might be expected... Then two more fish the next day then the catfish... I had been saving the fish to exchange at the pet store... Which we did as well as purchasing two different ones. Another catfish and a garami... The first issues we had east the fish getting sucked up into the filter... I out a mesh around the intake bit the flow was too great... The frog got caught in the mesh... But by the time we got him off it was too late... So we replaced the entire inside of the filter..... Changed part of the water... Over the next few days the fish continued to die... So here we r... Three weeks later..... I now know we have done everything wrong . I keep taking in water samples... They keep telling me they are fine... I've been doubt 20% water changes every day. Everyday fish continue to die... Today my pleco died.... I'm down to five small fish... Four glow and a guppy... Two frogs and two snails...I have two large sea shells, and four plants..... All in all I've murdered about 20 fish in the last three weeks... I am very sad and frustrated... I know I've made a terrible mess... I just need some advice on where to go from here...