Hi ljhannah,
It is not necessarily the gravel that is killing off your plants, and unless you are going to plant heavily a specialized substrate in not really necessary. After 2 years, you must have a fairly mlum rich bed.
By gravel I'm going to assume you mean the size of the pieces are larger than sand but smaller than pebbles, and about three inches deep.
Plants can store food and nutrients, this is called luxury uptake. When you buy a plant that has come from a plant farm it is well fed and has stored a lot of nutrients. I have seen plants last for months before they begin to slowly decline over a few more months. By the time the time you notice the plant is not doing well it isn't, leaving you to wonder what went wrong.
You need to look into a few basic things which will improve your success with plants.
Light: Rule of thumb needs to be at 2 watts per gallon of white fluorescent. In your case 60 watts. At this minimum you should be able to grow most plants.
CO2: Plants are 60% carbon and need a reliable consistent source to do well. Carbon in the tank in the form of dissolved CO2 from the atmosphere is about 5 parts per million. Plants need much more than that to flourish.
Seachem makes a product called "Excel". It is dosed as a liquid and is adsorbed by the plant. Once adsorbed it breaks down as CO2 in the plant cells and the plant gets all the carbon it needs.
Food...
NPK: Nitrogen, Phosphate, Potassium are the three essential nutrients (plant food) plants need to flourish.
Nitrogen: comes from Nitrates in the water, which is achieved by letting your nitrate levels go up to say 10 - 15 ppm.
Phosphate: Read the label on your flake food can.
Potassium: a good lfs may have a product for this, I know Seachem has a three bottle kit for all of the above.
Look into these basics, read through the posts in the planted tank forum, and ask questions.
Jay
