Sponge Filters

Ok so this may seem pretty stupid but here goes. I have changed over to sponge filters in most of my tanks. Now here's the question. How are they purifying the water with no charcoal?

They build up bacteria...bacteria is what actually detoxifies ammonia and nitrites/trates out of the water. Work much the same as undergravel/ wet-dry filtration just on an in tank scale

Ok so when I clean it I shouldn't get it really clean.

don't over clean sponge filters as some of the old bacteria needs to stay alive. rinse them in old tank water squeeze it out (not a hard squeeze). do not use tap water to clean sponge filters as it will kill off the good bacteria.

OK all thanks.

i like em. really great for breeding and putting them in q tanks.

Its biological filtration, not mechanical.
I tend to supplement them with small HOB filters in my older fry tanks.
I use them in main tanks to supplement the HOB filters for more biological filtration.

I also like them. I use one in a 100 gallon pond that has some goldfish and a yellow belly turtle. Keeps the water crystal clear, even outside in the sun. I plan on using them in my next tank.