Yes that may be a bit too many fish for a tank that size ... However
The most important thing in your case would be regular partial water changes.
Also read up on how Co2 levels are affected by water hardness. How hard or how soft your water is will really make a difference on things like; maintaining a consistent amount of dissolved Co2 and not having much variation, the ability to dissolve Co2 and how suseptible your tank is to a "crash".
Co2 will really help out the plants. Once the plants are doing well your fish will benefit also. Gold fish can do very well in low oxygen tanks, but of course do well with more. Also the plants tend to "eat" the waste. Goldfish are big poop producers, so the live plants are good.
I would recommend you go to 6500k lights, it a better color temperature for freshwater live aquatic plants. 8000k is really designed for saltwater tanks and certain corals. Freshwater plants are used to more "greenish" light since they are usually from regions with rainforest canopies. FYI. You also may consider at some point uprgrading your canopy with something a bit more powerful.
A few basic do's & don'ts about DIY Co2.
Don't guess, measure
your Co2 levels. Ph kit and water hardeness test are important. Learn the formulas for Co2.
Don't skimp. DIY items like gas separators, check valves, reactors are cheap to make and save alot of aggravation. And always remember that fermentation can create a whole bunch of pressure, use pressure vessels like soda bottles which are designed to deal with pressure. You don't want the thing to blow its top if you have clog and spew the mixture. Your house will smell like brewery for weeks.
Watch your plants. Look for an increase in bubbles coming out of plants. Its what you want to see.
Watch your fish. They tell you alot about your Co2 levels.
You may or may not have seen this link. This article gives you more than enough info on the science of DIY Co2 generation: http://www.qsl.net/w2wdx/aquaria/diyco2.html
I know because I wrote it.
Good luck and read read read.