You could try cutting the tip of the pen in the "T" at a 45 degree angle and see if that helps. With the cut end facing away from the water flow.
Also the air flow depends greatly on the water depth in your filter. The deeper the filter, the harder it is to draw the air to the bottom. Visually there will be less bubbles, but it still oxygenates the water.
As a side note I thought I remember hearing that you wanted like 1/3-1/4 of the pipes diameter filled by the ventri pipe. This forces slight back pressure and compresses the water. So once the water was past the blockage, it would allow it to expand to fill the rest of the pipe, and in turn creating greater suction to suck the air in. Which a pen would do in 3/4.
Edit: Just saw that its a 100g pond. Guessing you have maybe a 100-300gph pump. You might need more water flow/speed. If your piping from the pump is 3/4, then you can use a reducer and take it down to 1/2 or 3/8. This would force the water to flow threw at a faster rate.
Also the air flow depends greatly on the water depth in your filter. The deeper the filter, the harder it is to draw the air to the bottom. Visually there will be less bubbles, but it still oxygenates the water.
As a side note I thought I remember hearing that you wanted like 1/3-1/4 of the pipes diameter filled by the ventri pipe. This forces slight back pressure and compresses the water. So once the water was past the blockage, it would allow it to expand to fill the rest of the pipe, and in turn creating greater suction to suck the air in. Which a pen would do in 3/4.
Edit: Just saw that its a 100g pond. Guessing you have maybe a 100-300gph pump. You might need more water flow/speed. If your piping from the pump is 3/4, then you can use a reducer and take it down to 1/2 or 3/8. This would force the water to flow threw at a faster rate.
