Introduction: Cheap Filtration System for a SMALL Aquarium

OR COMPLETE BEGINNER'S guide to using a cheap and simple filtration system to maintain good water quality in a small (less than 5 gal) aquarium or fishbowl.

Introducing a cheap (around $4) small tank filter called "Discard-A-Filter" (I'd say one of these is good for an aquarium of up to 2 gallons. Use two and you could probably go up to 5 gallons. Note that you need an air pump to operate it, but these can also be quite cheap and you only need one, even to run two of these filters.) The filter is so cheap because the manufacturer expects you to just toss it in the trash every month and buy a new one (which would actually end up costing you quite a bit over the life of your fish). However, and this is not because I'm so eco-conscious (I'm reasonable: I don't go overboard, but do try to limit unneccessary waste when I can), the idea of just chucking a relatively large, sturdily manufactured object just rubs me the wrong way. It doesn't help that I happen to know that the only thing about the filter that stops working is the activated charcoal (and a smaller quantity of zeolite, which you can also, optionally, add), which absorbs harmful chemicals from the water -- it's absorbed everything it could, and has to be replaced with a fresh batch, there's no getting around this one. So, in this instructable I'll show you what you need to do to keep happily and cheaply reusing this filter, as well as improving its function.


I'm going to be REALLY THOROUGH here and explain every step into the ground. It'll be so easy a caveman can do it ;) If you find that annoying, I'm sorry -- it's my first instructable, and I'm kind of OCD about explaining things... Just skim through and look at the pictures, you'll get the hang of it.

AN ASIDE ON SMALL AQUARIUM FILTRATION (skip this stuff if you already know about how filtration systems work)

Everything I've read about keeping aquariums always ALWAYS says that a small tank is a bad idea for a beginner (the reason for this is the fact that even a small amount of fish waste can drastically change the water quality when it's dissolved in just a little water, so you need to keep a careful eye on water quality, and understand the factors that affect it). But what if you just want a little fishbowl on your desk (or wherever), and don't feel like sinking over hundred dollars into setting up a respectable size tank (i.e. 10 gallons or more, and believe me, it will add up to that much or more, with tank, stand, hood, filter, air pump, gravel, decor, chemicals, etc). On the other hand, you don't want to just have a betta death-trap, where you change the water from time to time and hope your fish doesn't die. You can always shell out $30 - $80 on something like the Eclipse Explorer (prettier kinds are on the higher end of that range), and have a relatively complicated setup to clean and maintain to boot. Or you can set up a very simple and very effective filtration system in your existing tank for about $12. However, I must point out that a filter doesn't replace the need for frequent partial water changes.

One of the things that came as a surprise to me was the revelation that ALL filters ultimately work simply by creating water movement over filtration media. Various filtration media accomplish several different things: they remove large particles, they (this is called biological filtration) turn (very toxic) ammonia into (less toxic) nitrates and nitrites (you can read more about this at, and they can absorb toxic chemicals from the water (not all filters do this step, called chemical filtration). There are multiple ways of accomplishing this, and several different choices of each type of media, but the underlying principle is the same.

The "Discard-A-Filter" is a type of sponge filter, enhanced with the addition of charcoal and zeolite (these absorb chemicals, while the sponge both filters out large particles as well as serves as a home to bacteria which convert ammonia to nitrates and nitrites). Despite its uncomplicated construction, a sponge filter is very effective at what it does, as well as very safe for your fish (there's no suction, so babies or injured fish can't get sucked into the filter, or stuck to it). Because the water movement is driven by an air pump (basically, air bubbles displace water as they move up though the filter, and are, in turn, displaced by new water), the water is aerated in addition to being filtered. Also, because there's no moving parts, there's nothing to break (your air pump could still break but that's a separate issue), and, because there's no complex assembly of chambers and reservoirs, there's MUCH less for you to clean and scrub! In fact, this filter is so simple, you could easily make one yourself, just by securing sponge and charcoal over an air bubbler with some kind of mesh -- the only reason I went for the "Discard-A-Filter" in the first place was because it brings everything together in such a small, neat, visually unobjectionable package.


This is not a necessary step, and many filters do not accomplish it at all. Ultimately dissolved chemicals are better controlled by frequent partial water changes, which are necessary whether or not you filter the water at all. However, I find it helpful to have chemical filtration in very small tanks, because it helps keep the levels of dissolved chemicals from fluctuating too wildly.

Most common chemical filtration medium is activated carbon (or charcoal). It is very good for absorbing organic impurities (such as proteins, as well as very small particles). It does not directly absorb dissolved inorganic impurities, such as ammonia, nitrates, or nitrites (claims that it keeps these chemicals under control, are not directly false, but misleading: some organic compounds, like proteins, will create nitrates/nitrites when they decompose, so activated carbon helps by removing the organics before that happens). There are other media that do, directly, remove nitrogenous wastes. Zeolite does this to a limited degree (but be careful, under certain conditions it can release them back into your tank). Products such as Nitra-Zorb, or Seachem Denitrate exist specifically to remove ammonia/nitrates/nitrites, and some of these can even be "recharged" to restore their absorbing properties. When reusing the "Discard-A-Filter" you can choose to improve its filtration abilities by using these products instead, or in conjunction with, the activated charcoal. Regardless of what you decide, keep in mind, you still have to do water changes.


To SET UP filtration in a small tank:

  • SMALL Air pump (you WANT a wimpy one, same effectiveness but less noise, also, they are cheap!)
  • Discard-A-Filter cartridge (just use it for the first month)
  • Plastic/silicone tubing
  • One-way-valve -- I highly recommend getting one: spend a buck or two now to avoid a huge mess and a ruined air pump, plus a possible electric fire.
  • Scissors
  • (Optional, if your chosen tank doesn't have a straight vertical wall) Superglue. Also hold on to the package the filter came in, you'll need a small piece of clear plastic.

To REUSE filter:

  • Pen knife
  • Superglue
  • Rubber band (use a clear one if you can, it looks better)
  • Jar/box of loose activated charcoal (this will last you a long time)
  • (Optional) loose zeolite (you can mix in a little with the charcoal, no more than a quarter zeolite though)

To MODIFY the filter (make it a little quieter/more efficient)

  • Airstone (they are cheap, so get a pack of several, they will need to be replaced occasionally)
  • Needlenose pliers (for breaking glue/plastic)

Step 2: STEP 1 - OPTIONAL - (Setting Up in a Tank With Curved Walls)

I won't insult your intelligence by detailing the setup. You run the airhose from filter to pump - that's it. Just don't forget to attach the one-way valve between them, and make sure it's facing the right way, too. As far as I know there's no "correct" place to put it, but I'd keep it closer to the tank rather than closer to the pump, though far enough away from it to conveniently hide it, if that's an option.

However, if you want your fish in a bowl or vase, it might present a problem, because the filter is held in place by suction cups attached to its very straight back. Not to worry though. Pull out the suction cups (go ahead and pull hard, you won't break anything, they are just stuck in through the holes.) Cut out two little pieces of clear plastic from the filter package (Yay for making use of our trash!) Spread a little superglue around the holes, and cover them with the plastic, so the charcoal doesn't fall out, and no bubbles escape prematurely. Now glue the suction cups to the bottom of the filter (use the diagonally opposed corners for maximum suction cup clearance.) Now suction cup the filter in the middle of your bowl, and you are done! If you think that looks ugly, try surrounding it with plants, live or plastic.


Wedge a knife at the top of the clear plastic grate between the grate and the filter body (it's your best gap.) There's a lip around the grate, which will help it stay in position later, but right now it prevents you from wedging the knife anywhere else. Gently lever the grate out. Don't be too alarmed at the loud cracking -- it's superglued in place, but do be careful not to crack the grate, it's pretty brittle. If you crack the thing, or worse yet, snap it in half, don't despair, you can fix it (read on into the mods, and it will become clear.)

When you've pried out the grate, hold on to the sponge and shake the used charcoal into the trash. Now pull out the sponge and rinse it gently in the sink. DO NOT use soap! Remember, that sponge is full of bacteria that helps filter out ammonia! Just get rid of most of the chunks on top, and put it back. Also: IF YOU DO NOT HAVE GRAVEL IN YOUR TANK (another good breeding ground for nitrifying bacteria), consider skipping the rinse completely. By the way, the sponge may have to be replaced at some point, if/when it becomes too degraded, but don't worry, you can buy a large chunk of it cheap and cut a suitable piece (for a no-gravel tank, rinse the old sponge inside the tank and squeeze all the water out before tossing it and putting the new one in -- that way most of the bacteria will make it onto the new sponge.) There is, however, no reason to just replace it every month, and every reason not to (i.e. the nice established bacteria colony.) If you plan to modify the filter, stop here and go on to STEP 3.

Now, in the back there's a rigid clear plastic tube, leading air into the filter body. Pull it off the nozzle where it's attached (temporarity.) Put the sponge back in, into the lower part. Take some charcoal (and zeolite, or any other filter media that accomplishes chemical filtration) and fill up the top part, but don't overfill. Press the plastic grate back into place. Now put the rubber band around the filter body to hold the grate in place. Reattach the plastic tube. Rince the filter under running water to rince out any charcoal dust and particles too small to be held in by the grate. You are done, put the filter back into your tank and reattach the air hose.


So, either you broke that clear plastic grate, or you are not done playing god. Either way, read on.

Ok, now that you've emptied the filter body, pull out the second, blue plastic grate that's at the back. That should be easy as it's not attached to anything. It's best to use needlenose pliers, but you can do it with your fingers easily enough: snap off the top solid part, as indicated in the drawing.

As you may have noticed, the clear grate does not quite cover the opening in the front of the filter. We are going to change that. Put the blue grate on top of the clear grate. Pay attention here for a moment: you can either glue it to the INSIDE or the OUTSIDE of the clear plastic, each has its issues and benefits. It is (at least IMO) more aesthetically pleasing to keep the blue grate on the inside. However, it won't lay flush with the clear grate, because of the aforementioned tabs that keep the clear one snug in the housing. So what you'll have to do is glue them together on one side only, and let them sit at a slight angle to each other. Just be careful that the blue grate doesn't stick out past either of the side tabs, or they won't fit back into the housing.

If you've snapped or cracked the clear grate, you probably want to make sure the two grates are flush, and the clear one is reinforced with the blue one. You'll need to have the blue one on the outside for that.

Before you apply the glue though, extend the blue grate a little past the clear, so their total length is equal to that of the opening (measure twice here, just in case). If you want to keep modding, move on to STEP 4.

If you were just reading this because you broke the clear grate, you are done: go back to step two and finish reading about refilling the filter (unless you've already figured out how. Hint: you use the rubber band to hold the grate in place, and get that plastic tube in the back out of the way before you snap the rubber band into place)



Perhaps my pump is just too powerful, but I find all the bubbling and gurgling exceptionally loud. My small tank is in the bedroom, too, and along with the hamster working out in the wheel, the noise level is making it difficult for me to sleep. And no wonder this thing is loud (way louder than the air pump, btw) -- the nozzle that leads the air into the filter body is quite thick, the air is not diffused in any way, so the bubbles are huge and "lumpy", and make lots of noise on their way. So when I ripped the filter apart to change the charcoal, I immediately decided the bubbles needed to get smaller, much smaller. And what's with bubbles sliding up the back of the housing, anyway? Sure, the water gets displaced, and has to flow in though the filter media, but what if the filter media is compacted into lumps? What if water is flowing more though some spaces and less though others all the time? That would cut down on efficiency. The most important thing about any filtration system is how much of the surface area of the media is exposed to waterflow at any given time. It's the surface area where all the magic happens. So I decided to kill two birds with one stone... err an airstone that is. What it's going to do is produce smaller bubbles, which are going to run THROUGH the filter media, agitating it, not behind it. That'll make the filter both quieter and more efficient, though admittedly not by much.


Here's the exact same kind I used. It's very generic (and fortunately small enough to fit inside, too). Get a pack of several of them: they are VERY cheap, and they do, occasionally, need to be replaced, just like the sponge. How do you know if your airstone needs changing? It stops making as many bubbles. I suppose this won't be immediately obvious when you are cleaning the filter (assuming it's not already obvious, because there's barely any bubbling going on in the tank), but if you want to check, just submerge the bare airstone on the hose and look at it. If the bubbles are being generated all over the surface, you are good. If there's maybe one or two places where the bubbles are coming through, pop on a new one. I don't have a time estimate for you on how long it'll last, as I'd just completed my own mod last night. I'll update this (if I can) when I know more.


While the filter housing is empty, pull off the plastic hose in the back. You probably won't need it, so you can chuck it. Do hold on to the little clear plastic coupler on top though, you never know when you might need to splice two pieces of hosing together. Grab the top suction cup and pull it out, you want to use that hole. Also grab the little nozzle that the airline was attached to with needlenose pliers and twist (sorry, you probably will need the pliers for this, I could never have done it with just fingers). The thing is superglued in there, but I happen to know that superglue does not stand up to shear forces very well (that's why you are twisting, not just pulling.) I suppose the nozzle doesn't HAVE to come out, but it might let out stray bubbles, and anyway, it looks weird. You'll notice that the hole it left is the same size as the one for the suction cups. I decided that I wanted my suction cups centered, so I pulled out the bottom one from its hole, and stuck it into the nozzle's hole, but this is a matter of taste. Either way, superglue a little piece of clear plastic (from the filter package, or the airstone package, whatever's handy) over whichever bottom hole you aren't going to use (but leave the top one alone!)

It is up to you if you want to make do with just one suction cup from now on, or attach the second one someplace else. I opted for attaching mine a little lower than it was before, and in the middle. However, because it's not going though the filter housing anymore, its stem is going too long. Cut it off at the "neck" and superglue it.

Now take the section of airhose that was attached to the top of the filter before, and pass it though the upper hole into the body of the filter. Attach the airstone, so it is inside the filter body, dangling at the bottom.

Ok, put the sponge back in, into the lower part (make sure the airstone is dangling as low as possible, you want air flowing all the way up). Take some charcoal (and zeolite) and fill up the top part, but don't overfill. Press the plastic grate back into place. Now put the rubber band around the filter body to hold the grate in place. Rince the filter under running water to rinse out any charcoal dust and particles too small to be held in by the grate. You are done, put the filter back into your tank!


Your suction cups might eventually get crusty and dry and stop functioning. That doesn't necessarily mean it's time to get a new filter, though you can, of course. To "rejuvenate" suction cups (assuming a thorough scrubbing didn't do it), drop them into boiling water and boil them for 2-3 minutes, then snatch them out and drop them into some very cold water. Even if you superglued them to the filter, you can still do this: stick the whole housing in, just pull out the hose.


Discard-a-Filter is not the only manufactured disposable filter of this type. There are others, notably SmallWorld. I imagine there's a way to pry open and "recharge" those too, I just haven't tried them. Now that you know what's going on inside them, what SHOULD be going on inside them, and why, there's no reason you can't give it a try :)