Plant dying need help con fixing this?

i got a plant that was grown from a pet store well more like a just fish store. but i brought it it was grown pretty and the leafs were perfect. i had to cut some of the roots off because it would not fit. the plant i think is a sword leaf plant i think. it was good for about a week and a half and then leafs started to fall off. now i have only like 10 leafs left and its been about 2 months or 3 months. i just brought a new light yesterday and its a 20 watt. the size of my tank is 30 gallon. the light is meant for freshwater plants and fish. the leafs are all discolored and i have non of that like dirt stuff or bacteria stuff so what should i do? does the light have to take time and what should the levels be the guy at the pet pantry store wrote down the levels and he has like 10 fish tanks so im pretty sure their right but here they are:0.0 for amonia and algae, 6.8-7.2 for ph, 20-25 for nitrate and nitrite is well i dont know the numbers but i cant find it but i think its suppose to be like blueish purple? are these right is there anything i can do to help plant growth go faster?