Well, I was going to use 2 2liter bottles/1 smaller bottle(plastic btw) to create my CO2 system. It'll run off an air pump rated for 10 gallons and I also have the 2nd nozzle that is unused on my 75 gallon air pump.
I take it that you are meaning a yeast based CO2 system? They are not very stable. They may work okay for smaller tanks if you stay on top of maintaining them. I'm not sure what the air pump is for? The system will generate its own pressure. If you seal up the bottles very very good with silicone you could probably get away with a small ceramic diffuser. Other then that there is the ladder diffuser option, which is pretty bulky. Or you can run the line into the intake of a internal or external filter and hope the motor mixes it in decently.
It wouldn't do much, especially if you don't have the light to require extra CO2. Bubblers and surface aggitation are also a no-no with any CO2 injection as it will just drive the CO2 out of the water making injecting it pointless.
Ditto M31. You would need a lot of yeast production to supply a 75 with anything substantial. You would be better to work on an effective fertilization schedule and even then you would have to have the light levels to make that work.
I seems that alot of what I read says that even a small amount of CO2 won't hurt and will greatly benefit the plants. I suppose I could turn off the volcano bubbler, I only have it on to keep the floaters away from the HOB.
Turning off the bubbler will help. As JDM said knowing your lighting as well as any fertilization will help in figuring out if you need and would even benefit from CO2. CO2 is not a cure all, it is a single nutrient and usually the last element in a planted tank that will become limiting.
The volcano bubbler and the HOB are both going to actively remove CO2 from the water you would need a true CO2 system to combat using a HOB filter. As JDM also said it will take a lot of yeast and a lot of sugar to make an effect on a 75 gallon, followed by the upkeep that comes with a yeast system. Irregular CO2 levels are just as bad as low CO2 and can encourage algae.
I'm still not sure what you intended to do with the air pump but a yeast CO2 system needs to be tightly sealed. Silicone all connections very well and make sure bottle caps are on tight. The system will slowly generate its own pressure which is used to feed CO2 into the tank.
Yes I have used that same DIY system in the past for short time before getting a proper CO2 setup but it was for a 20 gallon tank.
You don't really require CO2 at that light level on a 75 gallon. A good fertilizer regime would likely give you better results then messing with CO2, as well as switching to a non-HOB filter and reducing surface agitation as much as possible.
You can definitely give it a shot and see how it works. If you feel like seriously moving onto CO2 watch craigslist for a cheap used CO2 tank/reg. These are often used by home brewers (kegerators) and can be sold quite cheaply compared to an aquarium system, sometimes hydroponic ones show up too. You would have to replace the output on the regulator with a hose barb for aquarium CO2 line but thats not that difficult. You would also need a inline bubble counter, needle valve, and check valve but you can get those reasonably cheap off ebay.
Typically CO2 is suggested when you are running 2WPG or higher of HO T5s or similar high intensity lights along with heavy daily fertilizing. Just don't be surprised if you don't see too much of an boost from your yeast system. A standard system requires very little care compared to a yeast system. Only refilling of the tank every 3+ months depending on the size. CO2 is usually pushed more towards plant propagation or very high light red plants. I've been using a system for about 5 years now and certainly would not call them a waste as long as the lighting and fertilizers are high enough to require it.
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