I have a sponge filter and am very pleased with it.(BIG room for those nice filter bugs!)Now, I have a brand which you can stack on top of each other to further increase the filter bug capacity. I`m planning to do this , but the uplift tube is quite long and if the sponges are stacked on top of each other the tube will be above water level . Would it make any difference if I shorten the uplift tube to the desired lenght? What is it`s function anyway?I don`t want to decreace the efficiency of the filter... <_< Any thoughts?

What is the brand of your sponge filter? Or if you don't know the brand, what does it look like?

With my sponge filters, I generally try to have the outflow come out right at the surface of the water, so that there is a stream of water flowing across the tank. This helps provide oxygenation

I concur with touchofsky.

The shortening of the tube should also help the gph a little so it does achieve a little better efficiency alongside the aeration.

Don't you think that feature is used for deep goldfish tanks???

I have experimented with and without the plastic tube sometime back. The result is the same in my perception.

i wish i could find the page (about uplift tubes) that i read but, in it, it states that the uplift tube should be 1 inch (no less than 1/2 inch) inch from the water surface. it has something to do with the efficiency that the water is dispersed out of the tube. if the tube is set right at the surface, it takes more of the inertia, created by the rising bubbles, to slosh the water over the brim of the tube. the 1 inch rule also helps with the fact of evaporation. if the tubes at the surface and some water evaporates, you get less gph as modeled above.