I had five minutes to spare and was bored. So I made a sponge filter. Here it is:
Take a bog standard fish flake pot. Take the sticker off the outside. This plastic is guaranteed to be fish safe because obviously it is intended to store their food
Take a piece of tubing/hose about 1/2″ – 1″ diameter. Cut four little notches at the bottom (you can do this with a knife or saw).
Put the tubing down to the centre of the pot and pour gravel around it until about 3/4 of the pot is filled. DON’T lift the tube out at this point because you will have to then take all the gravel out and it is a right pain.
Now you can put some sponge or filter floss on top to act as mechanical filtration. One of those sponges attached to a scourer will do – if in doubt, look closely at the sponge. If you can see lots of little plastic fibres you’re ok – if it is closed cell and rots over time (sponges made of cellulose) then DON’T use it – it will rot in your tank. If you wanted you could get the lid and drill a hole for the lift tube and some smaller intake holes, but I couldn’t be bothered because it is a 5 minute sponge filter – that’s what it is. I tried to make it as bare bones as possible. Here are all the pieces so far:
Drill a hole the same or slightly smaller than the diameter of your airtubing and poke the airtubing through it.
If your tube is wide enough, add an airstone. Connect to an air pump.
And there you have it – a perfectly acceptable five minute simple sponge filter for a hospital/quarantine tank. Here is my sponge filter in action, literally five minutes after the idea popped into my head:
First publication: April 2005 op Fishforums.net
Source: Aquarticles.com (no longer available)