There are risks involved anytime you're working with a system that's under pressure. You have to evaluate that, but there are things you can do to mitigate these risks.
In a DIY system, you can add a pressure relief or blowout mechanism. Basically, get a T-connector for an airline, and put a rubber cap on it. That cap will "blow" before one of your bottles explodes.
As for getting too much CO2
. Depends on your tank size, but I'd say that's extremely unlikely with a DIY method. In my 20g tank, running 2 2L bottles, I was struggling to maintain 25ppm. It's said that fish are fine until near 100ppm.
If you're concerned about getting yeast into your tank, make a water trap. Get a small 1L or so water bottle, and put 2 fittings in the cap and fill the bottle 1/2-3/4 with water. Put tubing on of the fittings inside the bottl that extends below the water line. Bring the CO2
in on this line, it will bubble in the water and expel through the other fitting out to your tank. If you use a clear water bottle, this can also serve as a bubble counter.
Will your pH skyrocket if it expires. Well, if by skyrocket you mean go back to your supply water's resting pH, the answer is yes. But, this isn't an issue. pH swings from CO2
injection do not affect your fish. This is why pressurized system owners are able to shut off their CO2
at night without the pH swing being a factor.
Monitoring production of your DIY system. If you use a water trap, you'll be able to see the frequency of the bubbles, so you'll know when it slows down or even comes to a complete stop. Or, you'll see the CO2
being diffused into the tank and can monitor it that way. Another way is by testing pH.
If you're going to go the DIY route, I recommend using 2 source bottles. If you were planning on using a single 2L bottle, use 2 1L bottles. On my 20g, I was running 2 2L bottles and couldn't get the levels I wanted, FYI. However, having a 2 bottle system allows you to change them out at different times, so you never go without CO2
injection. With a DIY system, anytime you release the pressure, it takes 2-48 hours to rebuild enough pressure to actually get any injection into your tank. By using check valves and a T connector, you can set this up so you won't lose the system pressure by only removing 1 bottle.
I think I answered most of your questions. Did I miss anything?
Others will give their opinions as well. I will state that I tried the DIY thing for a few weeks, and got overly frustrated by not being able to maintain the levels I wanted and went to a pressurized system. While it's a bit costly, I put my system together for around $160, and now I am able to maintain exactly what i want in the tank.