Could you provide a picture of the tank?
What is the gh, kh, ph and co2 of the tank. There is always Co2 in the water.
How about just trying a different plant. Four leaf clover would probably work. I have it growing in my low tech 29 gallon tank. I had 65 watt of PC bulb and 20 watt T8 bulb. Now just went down to 2 - 20 T8 bulbs. No Co2, dosed with flourish comprehensive daily and sodium bicarb when doing water changes.