It is near impossible to overdose CO2 with a DIY setup. It's even difficult to do with pressurized CO2. I've never gotten anywhere near saturation levels with my set-up.
As for making the sugar water. Use 1/2 cup sugar (must be real sugar, artificial sweeteners will not work) for every 1 1/2 cups water and shake vigorously until dissolved. You have to hydrate the yeast first before adding, so to do that, fill a shot glass about 1/2 full with slightly warmed water (to warm will kill the yeast), add about 1/4 teaspoon yeast and stir. Let this yeast water sit for 10 minutes, then dump the contents into the pop bottle with the sugar water in it. Shake the mixture up to mix in the yeast. That's it. Replace the yeast/sugar water mixture every month or the alcohol content will build up and eventually kill the yeast.
The tube I use is just regular silicone airline tubing, the same stuff pet stores sell next to the air pumps.
Fish do fine with CO2 in the water, it just can't get supersaturated or they will get a syndrome similar to SCUBA divers getting the Bends (bubbles in the arteries). This shouldn't happen dosing CO2 in this way.