I have LOTS of XY-380 sponge filters available, £4.50 each. I’ve sold hundreds already, they’re great for tanks up to 250L (supposedly 380L but I wouldn’t) although I’d use 2 if possible as you always have one spare to set up a second tank in an instant. They’re about 12cm by 12cm and are simply air driven. Especially good for shrimp tanks. You simply won’t find these cheaper, not o line and especially not in shops.