This is a really cheap and easy instructable for anyone who owns a fish tank or a small decorative pond.

We have a goldfish (named Lilly) who lives in a cold water tank on the window in our main bathroom. As we approach summer, the tank is catching more sun and the water gets very murky every few days, this is not good for Lilly. Currently, the only way for me to clear up the water is to change it every couple of days, also not good for Lilly due to rapid changes in water chemistry, temperature and the stress of the water change.

If you are interested in more information about owning a goldfish, have a look here

A filter is really the answer, this will clean the water of particulates while leaving the temperature and chemistry alone and Lilly gets to enjoy the new current it creates. The problem is that I could only find mains AC powered pump/filters, and as my tank is in the bathroom, there is no AC.

THE SOLUTION!!!! Build a battery powered system and drop it in.

Step 1: Tank Filters

There are lots of different kinds of filters for fish tanks, some used sand, stones, and charcoal, others use foam filters.

All work by pushing or sucking water through the filter material in order to clean the water of suspended solids (causing water turbidity) to create clearer water. The charcoal filters actually perform some purification from other chemicals.

I am going to build a foam filter with a suction pump, this creates a vacuum in a space above the foam filter, sucking water through the foam and trapping solids.

Step 2: You Will Need:

For this instructable you will need:

A submersible pump (get one here)
A battery pack (my one is from an old mechano set)
A small lunchbox
A foam sponge (I got the message type with the rough part on one side and soft on the other)
4 AA batteries
Solder
Hot glue
heat shrink

Step 3: Filter Chamber

Start by cutting the sponge to fit neatly inside the lunchbox, I got lucky that cutting the sponge in half fit perfectly, the other half can be stored as a replacement filter when this one gets too dirty.

Drill a series of holes in the back of the lunchbox to allow the water to get sucked in.

Insert the foam filter and admire for a second.

Step 4: Set Up the Pump

Get the placement of the pump correct so that the outlet will be outside the housing.

I mounted mine inside the lid so I could open it up and change the filters.

The hole has to be tight so that water doesn't get sucked in the front and bypass the filter.

A 6.5mm hole was really tough to push the tube through making a great seal.

Step 5: Clear Space

The pump is going to take some room inside the chamber so you will need to remove a section of the foam as shown in the picture.

Drill a small 2.5mm hole in the side of the box to pass the wire for the pump out.

Step 6: Wiring

It is all important to remember that this unit will be underwater and also remember that electricity and water don't play nice.

I made some twisted joints between the pump and the power pack, I then soldered them to make a strong joint.

Rather than just heat shrink, I first coated the joints with hot glue to seal them, I then put some heat shrink over the top to add an extra layer of waterproofing and add some more strenght.

Step 7: Testing

I made the pump earlier today and this evening dropped it into a fairly dirty tank.

The good news is that the waterproofing held.

The bad news is that it floated, even when full of water.

I tried to add some ballast with stones from the bottom of the tank but still no, in the end, I put a big rock from the bottom of the tank against it to pin it in place.

The pump is running great and I can see some of the dirt getting caught but from experience, it will take a day or two for the water to clear completely.

I will try to keep an eye and report back in to let you know how long it took to clear and how long it lasts on the batteries.