Why does the Venturi tube over flow

[fish]

I had a 10" venturi tube on a 75 gallon skippy filter. For the first few months the water in came up about half way up in the tube. Then it strated to overflow the tube. I changed to a 24" high tube and the water overflows that height. I drained the tank thinking that there was a buildup of to much gunk in the bottom. The water still overflows the 24" tube. Any idea why the water overflows 24" high?

 

[fish]

I had a 10" venturi tube on a 75 gallon skippy filter. For the first few months the water in came up about half way up in the tube. Then it strated to overflow the tube. I changed to a 24" high tube and the water overflows that height. I drained the tank thinking that there was a buildup of to much gunk in the bottom. The water still overflows the 24" tube. Any idea why the water overflows 24" high?