Plants for a small tank?

I have a small 2.5 gallon tank and a 30 gallon tank, both filled with guppies. I am looking to put live plants in both but I am not sure what to start out with? What would you rcommend? Also I can't afford to a lot of money, so maybe something not too spendy.

low light plants and maker sure you have a 6,500k to a 10,000k bulb so the plants will photosynsize. cryptocornye sp., different types of java fern, different types of anbuis. dwarf sag.. different types of vals.