anacharis doesn't do well with CO2 imho, it grows so fast that it becomes thin and starts putting roots out all over the place - of course, this is also with medium high light
i've had better luck growing it at around 1.5 wpg (fluorescent), grows much slower and fuller
I don't think you should add any fertilizer for it, it should do fine with just water changes and fish mulm ... if your water is very short on iron, maybe some "root tabs+ iron" in your gravel will help
how much light are you using, and what kind is it? ... 2.5 gal probably has a small 7 or 10 watt incandescent tube light inside the hood? ... incandescents aren't good for growing plants
for plans on diy co2, just use google, type diy co2 and you'll get plenty of help, but again, I don't think co2 is your problem