Not sure who would give you that advice. Unless the sponge filter is made out of carbonates, it will not be increasing pH.
In fact, the natural process of the beneficial bacteria on your filter will decrease pH. The BB takes in H2O, uses the O (oxygen) and releases H (hydrogen). A net increase of H. And we all know that when there is more hydrogen in the tank ... pH drops.
The only things that might cause the pH to increase in a tank is shell grit, rocks, or your source water is naturally high in carbonates.
Do you have rocks in the tank?
Do you have any shells, corals, cuttlefish bones, etc in your tank?
Try taking half a cup of substrate from your tank, and test it. Test pH of that distilled water. Add distilled water to that cup of substrate you removed. And then test the pH periodically to see if it increases over a day.
Do you use any other sources of water apart from the distilled water?