Congrats, I recently built one too! I used two 2L bottles, joined with a T. The recipe I used is 2C sugar and 1/2 Tbs yeast in each bottle, and it seems to be going great. It's not been going for a complete cycle yet, so I can't tell you how long this will last, but it's putting out a nearly constant stream of tiny bubbles. There has been noticable growth over the three days it's been running. The length it goes will depend on three things: your yeast, the temp of the bottles, and the amount of sugar in the bottles. Yeast like temps in the mid 70s F, and will do best there. Different strains of yeast have different growth rates, and some can tolerate more waste (alcohol ) than other strains.
You COULD do it that way but I think I've figured out a better method based on looking at various commercial CO2 injection systems. One you could just go buy the co2 diffuser that come with most of them and feed your airline to it. The option I currently use however is to sink a small container (shot glass in my case) and weigh it down with some rock or something along those lines. Run the airline under that and let the co2 bubble into the upside down glass. Just make sure its well anchored or itll float away when enough co2 builds up. This way the co2 is trapped within the water and has more time to diffuse. Excess will work its way out of the glass and bubble up normally. Works wonders for me.
matt, you need a better filter. An internal filter is alright for small, lowly stocked aquariums but yours is over stocked. You will be needing a better filter, a powerfilter would be best for that size tank, you might need double or even tripple filtration.
Im not gonna be able to get a picture up here anytime soon. Shouldn't really need one anyway...fill the shot glass with water...submerge it...turn it over place the airtube so it bubbles up into the shot glass...and weigh it down. piece of cake.
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