I haven't heard/seen much (if any at all) success using an HOB to diffuse their CO2. I assume that the waterfall output of the HOB releases most of the CO2 into the air.
Tank looks good. Have you looked into rootcaps/substrate fertilizers? Also, check out RootMedic OneStep. It's now 14usd for 500mL, not sure on the shipping. Seachem Flourish is about 10-13usd at most large retail store.
Currently out of the hobby
Forum lurking every now and then