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 http://en.wikipedia.org/wiki/Aquarium#Nitrogen_cycle), 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.
A NOTE ON CHEMICAL FILTRATION:
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.
Step 1: WHAT YOU NEED:
- 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.
- (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.
- Pen knife
- 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)
- Airstone (they are cheap, so get a pack of several, they will need to be replaced occasionally)
- Needlenose pliers (for breaking glue/plastic)