I have a used 90 gallon that I filled halfway. I used the following to create my own filter.
1) Aquaclear 70 Powerhead
2) Old aquarium gravel cleaner
3) filter floss
4) ceramic rings
5) Old breeder box
6) cheap rubbermaid container (the small kind for salad dressings and the like)
A) Take off the hose from your gravel cleaner and attach one end to outflow of powerhead
B) put ceramic rings at bottom of breeder and attach breeder with suction cups just above the level of your water (waterline)
c) drill a hole in the lid of the rubbermaid container just shy of the diameter of the other end of the gravel hose and force other end into hole of lid about a 1/2 inch. (you may opt to silicon here but I didnt have to
Next, drill or use a heated tip of screwdriver (uisng stove or flame) and poke a bunch of holes all around the sides and bottom of the container itself.
D) Pack/Fill the rubbermaid with filterfloss or sponge (your pref)
E) Place lid back on rubbermaid container and set atop the layer of ceramic rings
F) turn on powerhead.
You can easily change media by taking lid off,replacing filterfloss,putting lid back on.
Here are a few pics. to better show.
it has been working well and am in process of building a diy background to have more of a waterfall effect.(to hide the tubing,DIY filter and all)
I would gather some of the more experienced aquarists here can offer a better solution then what I have, or perhaps add to it with better suggestions. maybe (2) for your size tank?
I can post shots of that when I am done if interested.
the reason I like this DIY version over say one of those storebought reptile terrarium filters is of course the fun in saying "hey I did that" but also because I can adjust the flow/filtering rate, its easy to clean and was affordable (since I already had all the stuff lying around not sure about your situation) and i did the whole project in less then an hour.