You are going to laugh at me. Rotten mulch... I recovered a whole anoxic pond as a volunteer for the Colorado Wildlife Division using it.
Now, big water bodies aside, I'm not sure in a water tank. Being a biologist, I know there are a few things plants need in order to grow; sunlight, nutrients, CO2, O2, and water. You can't really remove the dissolved CO2 or O2 or the water in a cheap way. You are left with either blocking light from reaching it or do something about the nutrient in the water. Are you using tap water? Using filtered/deionized/distilled water makes a huge difference. Can you paint the tank? That should reduce the amount of sunlight it gets. If not, try calling a fish tank store and ask what they do for it. There should a product to inhibit growth.