HOBs are fine. No need to upgrade the filter.
Not everybody is looking for crazy growth and lots of maintenance/trimming. Moderate growth and helping to keep algae at bay is more than sufficient for most people. Just changing the filter is not going to help. It does not address the issue, not to mention that he isn't even running CO2 at the moment.
Just keep the water level close to the filter return, as others have said, and you will be fine if you decide to go the DIY CO2 route.
Excel does melt Val according to most people that have it and have dosed Excel.
The pinewood air diffusers work fine for DIY CO2. Not a lot of back pressure and you get fairly small bubbles. Fluval makes a small ceramic diffuser that works well with DIY CO2 and doesn't look out of place in smaller tanks. The pressure is a bit higher, but the bubbles are a lot smaller. 12/16/20 oz soda bottles work well for smaller tanks. 2 liter bottles have a lot more surface area and will produce more CO2. I've used 2 liter bottles on a 10 gallon tank without issues, but it was heavily planted. I usually add some molasses to slow the fermentation and draw it out an extra week or two. You don't have to get too fancy with it. A soda bottle (designed to hold pressure, somewhere up to 100psi I think), some air line, a check valve (which you already have), a diffuser, and some suction cups.
Don't fill the bottle more than two thirds or so. The head space will allow for some foaming without carrying it into the tank.
Put some sugar in the bottle and some luke warm water and shake it up. Add a little bit of yeast (I get around ten batches in a 20 oz bottle from a single packet of yeast). You aren't brewing beer, you aren't looking for a vigorous fermentation, you want to draw it out over an extended period. Like I said, I add some molasses to extend the fermentation by adding some complex sugar strains and nutrients. Some people have complex recipes. I keep mine fairly simple. Like with other aspects of planted tanks (or any hobby really) you can make it as simple or as complex as you want. It's all up to how much money or time you want to invest and what type of enjoyment you get from it (more time working on the tank vs. more time spent watching the tank and it's inhabitants).