[trenac]

You can run your C02 24/7 if you wish. As long as your fish don't gasp at the surface, you'll fine. You could also run a bubbler at night (or any extra surface agitation) to get extra airation in the tank.

You might want to look into a Rena Filstar filter instead of a Fluval, they are much better filters IMO.

Your fine with the fish load you have now. You may want to think about adding to your small school of Neons & Golds.

 

[trenac]

In a planted tank the plants act as filtration, the heavier you plant the tank the better off you are. Plus if you buy a filter rated for say a 45 gallon tank, you will have plenty of filtration to take care of a heavier fish load. Since your Golds are already 2" & seem happy as they are you may just want to add a larger school of Neons. But adding a couple more Golds want hurt either, it's up to you.

Most people do not run carbon in their planted tanks all the time, only to remove medications or cloudy water etc.. Carbon can remove nutrients needed by the plants when new, but after a few weeks begins to loose it's ability to do so & will act as another form of bio-filtration.

Your tank is looking good already. Looking forward to seeing it fully planted.