where to find DIY co2 system?

Question

how often DIY co2 should use to lower PH level

Answer 1

You'll find a DIY CO2 system in your home - the DIY stands for Do-It-Yourself.
Mind you, you can purchase pre-made "DIY" reactors, which are basically a tube/bottle with a hole at the top for an airline that leads to either a powerhead to break up the bubbles, or a plastic zig-zag fixture which forces the CO2 bubble to travel a long way, so it can be absorbed by the water.
If you want to make your own, it is very simple. Take a 2litre coke bottle, rinsed. Take the cap, and drill a hole in it that is slightly smaller than the airline. Cut the end of the airline at an angle, poke it through the hole, grasp the end with pliars and pull the airline through about 2". That way, it is perfectly sealed.
Inside the bottle, put 1 litre of warm water and one cup of sugar, and allow sugar to disolve. Then add 1/2 tsp of baking yeast. Close the bottle, and attach the other end of the airline to whatever you're using to disperse the CO2 in your tank - either a powerhead, one of those plastic zig-zag things, a CO2 airstone, or a CO2 diffuser made out of glass (you can find them on e-bay). Then put the diffuser in your tank.
You'll leave it on 24/7; the yeast/sugar will continue to produce CO2, after all. You'll need to replace the yeast and sugar about once a month.

It won't lower your pH level very much, though. Maybe one point. If you're using CO2 to lower your pH, you'll need to use something stronger, like a pressurized CO2 canister, or a carbo-block - but be very careful, it can crash your pH from like 10 to 5 in an hour (happened to me, killed one of my angelfish)

Answer 2

This looks like a typical system:
http://www.aquabotanic.com/DIYCO2.htm

the biggest problem will be getting enough pressure to inject it under the water by more than a few inches.