I'm not sure what the wattage is of your light, nor how big the tank is. I am sure that you need more fertilizer than Plant Nutrition+. You need to be dosing nitrates, phosphates, potassium along with the trace elements. The undergravel heater will probably work to keep the water from getting too cold, but that is all it will do that is beneficial. Try reading the posts in the Estimative Index topic here to see how you can fertilize and be sure you are doing a good job doing so.

If your light is a high enough wattage to give you over 2 watts per gallon you will definitely need CO2, and a drop checker will eliminate a lot of the guessing about whether you have enough of it in the water. With all of that you should be able to grow plants and avoid serious algae problems.

Edit: I did some research: your tank is about a 55 gallon tank with about 90 watts of T5 light. You do have a high light intensity if the two T5 bulbs have individual reflectors, and possibly even if they don't. HC should grow fine with that much light, but you do need adequate, complete fertilizing and a good level of CO2 in the water - about 30 ppm of CO2.