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 online and especially not in shops