Let me guess - you're using a scintered glass diffuser? DIY CO2, especially if using airline tubing (which is somewhat permeable to CO2) are seldom up to the task of forcing CO2 through these things. Better get a reactor or a fine airstone (limewood or certain disposable ones work pretty well).
For DIY, it's best to have a shorter run of tubing. Having the tubing coiled a few times above the tank's waterline is a good way to help keep tank water out of your CO2 bottle (or airpump, but that's another thread) if you keep the bottle below the tank's waterline. The most important thing is to keep the bottle somewhere where it won't tip over. Pumping yeast/sugar brine into a tank is absolutely no fun. Some use a gas trap to minimize the chances of this happening. The easiest way to check your rig's seals is to submerge the bottle in a sink full of water or to coat it with dilute soapy water and blow into the end of the tubing that will go into the tank. Any bubbles seen? If not, you're OK.
To add another ~1 cup, use 3/4 C water. 1/4 C sugar.