I'd advise to get rid of the sponge. They harbor billions of bacteria.
What do you use instead of a sponge? I keep a sponge in the freezer that is only used with oxyclean on buckets/spoons etc..
Here is my info on the aftertaste, as I have it too.
I describe it as a bad bitter, it starts ok, then the after taste is ick. Almost slightly tannin like too..
I've gotten this with extract, and all grain. With Bottled spring water, and with filtered water. Got it doing partial boil on the stove and full electric boil on all grain.
Got it when left in primary over a month, got it after a 3 week primary.
Fermentation is in a 58-60 basement cupboard, so it never gets hot.
My mas temps are good, never high, sometimes low. Gravity all works out great. etc etc etc.
Got it on a pale, an english brown, and an oatmeal to a bit. Liquid yeast, dry yeast.
My only things to try next, better aeration and pitch temp...
I pitch between like 70 and 78 then drop to 58-60.. Think yeast is starting too soon and temp isnt dropping?
Seen a lot of people mention pitch temp should be equal or lower than ferment temp.
So I think that is the main cause of this bitter flavor. Going to pitch at 58 or less and test on a recipe I have already done.