java fern is so low maintainence, I'd say c02 isn't necessary at all. Hornwort info (from the web)

--Quiet water of lakes, ponds, marshes, and streams, where it is common, often abundant.

--A good pond oxygenator, it usually grows submerged in the water but is sometimes found floating on the surface.All Ceratophyllum species are obligately submerged aquatics and cannot tolerate periods of emergence.

--Typically neutral or alkaline waters.

--Adapted to grow at relatively low light intensities and therefore may be expected to tolerate shade better than most submerged aquatic plants.

--Thin submerged leaves and large surface to weight ratios make it an important habitat species for many aquatic invertebrates.

--Commonly found in plant communities that include species of the genera Elodea (Waterweeds) and Potamogeton (Pondweeds).

Care Level: Easy
Light: Natural
Placement: Floating
Tank Conditions: 59-86°F; pH 6.0-7.5; KH 5-15
Propagation: Side Shoots
Max. Size In Aquarium: Up to 1' 8"
Color Form: Green
Origin: Worldwide
Family: Ceratophyllaceae

I hope you don't have the goldfish in that 10 gallon. What size are the goldfish? Cheap or not, they won't survive long if the tank is too small.

Yes, the goldfish are in my 10 gallon tank, they are still very small (1.5") and there are 8 of them. I know the tank is too small, but they survived last year and hopefully next year I will get them something bigger to overwinter in.

No need for CO2 with only 2W per gallon (make sure it is full spectrum for the hornwort). Both plants are pretty durable the hornwort may appreciate a little more light though.If it starts to look unhealthy maybe increase intensity (light). CO2 benefits all plants, but like the above profile says for hornwort alkaline to neutral, CO2 usually acidifies the water. Better choice unless you increase the wattage is a product like flourish excel that produces carbon. In my 29G I use a DIY fermentation CO2 generator. The ones you buy at the store are somewhat good but overpriced. They also don't include a seperator to remove by-products from the crude CO2 produced from a yeast and sugar mixture. It can release into the tank and become toxic. Do alot of research on them before you try it. If you want I can give you a few links if you deside to try it. I made my DIY CO2 for under $20 (also had some stuff on hand) There are plenty models in articles on the net just do a search. I think it is well worth it as long as you understand it.