I use 2 cups sugar with a 2 liter bottle, lasts much longer. I get about 17 days of good bubbles out of that.
How did you attach the tubing to the bottle cap? Check that for leaks.
My best guess is that it used up all the sugar. The clumping/sinking is normal.
What kind of filter do you have on the tank? If possble, run the co2
tube right into the filter intake. it smashes up the bubbles and you get better adsorption of co2
ito your water.
If that is not an option, I'd get the Hagen co2
diffuser (around 10 bucks I believe). you don't have to purchase the whole system, just the co2
-proof tubing, and the diffuser (essentially a bubble ramp, the bubbles have a long time under water, and the co2
Either of these methods is way better than a standard airstone, which is very inefficient.