HOB filter options?

[stoddern]

hey guys first let me say thanx for being so helpfull, I'm glad I joined this forum. so for my first post my question is, I have a 20g I thinks a long since slightly deeper then my 10g by maybe an inch and a marineland penguin 150 HOB, it has 2 slots for filter cartriges but I only run 1 I would like to put some kind of bio material in the first slot and not sure what type of material to use (can I get away with scotch brite?) also I keep reading here that alot of you don't use carbon, why? my tank is planted and I plan on getting more plants I also have a tri. sponge filter in the far back corner oppisite the HOB it is buried in gravel to top of sponge with a single layer of gravel on top to hide this runs off an air pump, under the gravel is Seachem Flourite Plant Substrate, my goal is to over filter (preferably with bio, bacteria) so I can over populate, mainly I want to know if I need to use carbon? if so whats a good brand of loose so i can just use a refillable bag, if not great, and what would make a good bio filter to add to the HOB, also I feel my bacteria isn't growing as well as it should, when I set up the tank I planted it and let it cycle for 2 weeks before adding fish I then added 3 danios, 2 iddity bitty minos, 1 blue gouami and 2 golden gourami, been about 2 weeks thus far been doing 30% water changes every 5-6 days and declorinating as directions on bottle to bucket of tap water before adding, letting it sit for about 20min. then adding recomended bacteria supplementand again waiting roughly 20min. before adding to tank both declor and bac are from API do I need to just wait longer for the bacteria colonies to build? I have an aqua-glow flourecent from hagen coming and and freeze dried blood worms aswell, my heater is a self contained steady 78degree cheapo from tetra, I also have on order a power strip that has 2night time and 2 day time plugs aswell as 8 alltime and is digital 7day programable was gonna use it for the light and maybe my sponge filter/bubbler, wish I could tell you what my chem lvls are but the test kit is also on order aswell as API ammo lock, and I as for what I want for more plants I plan on getting some kind of floating carpet and maybe some java moss to partially carpet the bottom I also plan on getting a bristlenose pleco but want my bacteria to come up abit before I add that kind of bio load, also last question I swear hahahaha what do you guys feel about the new cO2 self contained yeast units? I would like to have some co2 for the plants for health but don't want to fus with 12g cartriges or going full regulator style was thinking of doing something with an old 12oz co2 bottle from paintball since I had to give it up in order to do nitro R/C, any way do I even need co2? sorry I tend to ramble alot.