Alright so we're growing aquarium plants in a 100 gallon tank, so far we have a variety but a couple are turning brown even though the tank has a lighting system, and the substrate is gravel. So far the 'Petco' experts' say that everything should be fine, and I'm also keeping the water at around 80-82 degrees F. Now I bought some aquarium plant food, and dropped 12 tablets, per instructions, into the tank, my main concern is how often do we put this kind of fertilizer into the aquarium. The box doesn't state anywhere how often to do it!
I would really appreciate any advice on this matter and any other helpful tips that can help the plants grow faster, lusher, and greener! Thanx in advance!

When you say "gravel" is that just plain rock sort of gravel or is it proper plant substrate? If it's the rocky stuff your plants will not grow as easily compared to using dedicated/specialised plant substrate. 5d2d.com Pets Questions and AnswersPets Questions and Answers

What type of plants are you talking about? Are they low/medium/high growing plants? 100 gallons is a decent amount of water - the depth of your tank might play a role in how much light reaches the low/middle growing plants.

Have you considered investing in CO2 injection? I have had a successful planted aquarium in my past and I used CO2 injection in combination with lighting, plant substrate and liquid fertilizers. I added the CO2 injection last to my tank and boy did I see a huge difference in less than a week! I won't mislead you - it is quite expensive, but well worth it if you are serious about a planted aquarium.

As for the frequency of fertilisation - that will depend on how many/type of plants you have and how often you do water changes. I used seachem liquid fertilisers and used them at different intervals at every water change. If you look at Seachem's website they have a "calculator" to how often, how much and what fertilisers you should try to be adding for a healthy tank....i didnt follow it strictly as they will obviously get you to dose more to use more of their product.

Lastly - I quite often used this forum for all my aquarium plant questions: http://www.aquaticplantcentral.com/ here you will find heaps of people who are more than willing to help - there might be a few posts already that answer your question.