It could also be that the new sponge filter isn't cycled. Take some gravel or other substrate from a mature tank, & place it on top of the sponge. This helps to transfer nitrifying bacteria to the sponge, and uses the gravel as a temporary bio filter.
I know thiss is a little off topic but you have a nice tank and your fish seem pretty healthy too.
I guess the water is getting better cause it isnt very green in their