Fish Advice Needed?

we had our tank last November it is 3ft everything was going well we bought 6 Zebra Danios and then 3 Molly's who had a baby Molly who grew really well. A few months ago we bought a Salifin Plec and we introduced 7 Neon Tetra's. Within a few days 4 died and now we have 3 but they are doing really well. Then one of our Molly's died and there was nothing visably wrong with her. The male molly and baby molly had prevoiusly started to get a white cotton wool type fungus which i put medication in the tank for and it improved. After the female molly died they began staying at the bottom of the tank. I took a water sample to the fish shop she said the amonia levels are slightly high so i did a water change, but nothing else seemed wrong. I have been changing the water weekly. Today i have found baby Molly has died. Myself and my husband are both really upset as he had become our favourite. We have a collection of snails which came on the plants we bought could this be a problem?please help