Science Buddies: "Ask an Expert"

I am experimenting on this question- Which concentration of fertilizer-2%,4%,6% has the worst effects on the Amazon Sword Plant (freshwater aquarium plants)? I just finished the control with no fertilizer and the results were not what I expected. The plants are supposed to grow about half an inch a day according to articles I read. The plants only grew about half an inch in three weeks. I had a ten gallon tank at 70 degrees Fahrenheit and a 15 watt Grow bulb on for 12 hours a day.There was gravel at the bottom and a water pump on. The fertilizer acts as a pollutant. There is nothing else in the tank besides the plants and gravel.I don't get why they didn't grow. Do you have any advice on what I should do?

Dear Ama,
I read some info about the plants. You might need about 20 watts bulb as it is a 10 gallon water, ( 2 watts per gallon), and also want to check the ph/alkalinity of the water. Also your question is about the fertilizer effect so if you use the 2%, 4% or 6% your growth of the plants will be accordingly.Control plant without fertilizer may not have grown because there is no fertilizer to boost its growth. I hope this is helpful.
Jay