To filter, or not to filter, why you might need a filter, and what filter to use.
Unlike a lot of people who give out betta advice on tumblr, I have quite a bit of experience keeping betta in unfiltered bowls. I maintained all the displays for work (unfiltered bowls between .5 and 2 gallons, plus ~60 fish in cups at any given time), and watched fish in various sizes of enclosure suffer health consequences based on being in those containers. I have firsthand experience when I say what my minimum size is for a healthy betta.
I'm going to make a controversial statement here when I say that I don't think betta fish absolutely need to be in a filtered setup. One of my fish has lived in an unfiltered/100%water change weekly 2.5 gallon tank for several months now, and is healthy. He's not just suffering no consequences, he is actually healthy. When I got him, he had developed severe swim problems due to being in a high-ammonia environment for months. He still has problems floating and swimming, and would not be healthy if he was in a tank with a water current.
On the other hand, I had a very large betta named Steve who was living in a 1-gal bowl when I got him. I moved him to a filtered 10-gal tank and his fins doubled in length, he developed iridescent scales, and his pectoral fins went from being stick-thin to long and flowing. Steve couldn't live to his full potential in a small bowl.
What I'm trying to get at here is that blanket statements about "this is acceptable" and "this is not acceptable" don't apply to all fish. Environments that are good for some fish are not good for others. It's important to learn what a healthy betta looks like and acts like, and to compare those betta to you to determine if your environment is meeting your fish's needs.
I'm going to use the word "tank" for any enclosure that you have your betta in. This information applies to tanks, bowls, plastic shoe boxes, wherever your fish is.
A couple myths: plants, shrimp, and snails do not filter your water. Shrimp and snails actually add to the bio-load of your tank, and produce the same waste that your fish does. Plants like peace lilies and lucky bamboo are not meant to live underwater. Parts of them will decay and detract from your water quality. Marimo balls only grow .5cm a year, making them too slow of photosynthesizers to remove any wastes in your water, though they don't decay the way peace lilies do.
This is not to say that any of those are bad for your tank. All I'm saying is that having those in your tank does not mean you need to change your water less frequently.
I went over this in another part of care, but there are a few things that also contribute to your water quality worth mentioning again. If you are feeding your betta food with a high content of plant fillers and a low content of meat products, your fish will poop more frequently and produce larger poopies. In a small environment, this will make your water quality deteriorate faster. If you overfeed your fish, uneaten food will stay on the bottom of your tank. This food will decay and contribute to poor water quality.
Also, all betta tanks should be heated so that the water stays between 74-82 degrees. Living in an unheated bowl will drastically reduce your fish's life span by months or even years; you're literally slowly killing him. I'll go over heaters in another post, and just now leave you with the advice to research any heater on Amazon before you pick it up. Some heaters are unstable and have a tendency to consistently overheat and kill your fish.
So, to filter, or not to filter?
Why do we filter?
When your fish breathes, pees, and poops, it creates ammonia. Fish cannot live in an environment that is high in ammonia. If you have a filter, you grow a colony of bacteria that eat the ammonia and produce nitrate. This colony is called your biological filter. Nitrate is much less toxic to your fish. A good filter colony makes sure that your fish live in an ammonia-free environment.
If you have a functioning biological filter, you do not need to change all of the water in your tank every time you change your water. If you do not have a biological filter, you will need change all of the water every time you change the water. Because you do not need to change all the water every time you do a water change, you can have a tank that's 10 or 20 gallons without using 20 gallons of water every week to maintain it.
Why would we not filter?
You need at least 2.5 gallons of water in order to support enough bacteria to make a functioning biological filter. This means that even if you have a filter running, if your tank is less than 2.5 gallons, you do not have any biological filtration.
Betta fish come from still (not small, not dirty, still) water and don't like moving water. Therefore, if your tank is too small to support a biological filter, it is better to not have a filter running in a betta tank.
Betta fish do not produce much waste, and they get oxygen by breathing air at the surface. Because of this, they do not need water aeration, and a 1-gallon tank will not reach dangerous levels of ammonia in a week, so it is possible to keep their water livable without a filter. Other types of fish cannot live in conditions that betta are comfortable with.
What about other kinds of filtration?
Technically, a filter can provide three kinds of filtration: biological, mechanical, and chemical. The really important one is biological. Your biological filter keeps your water breathable for your fish. If your filter doesn't provide biological filtration, it's pretty useless for betta.
Mechanical filtration takes all the floaty particles of fish poop and uneaten food and plant decay, and it holds all those pieces in one place (usually on the filter sponge or filter pad). No matter how good your mechanical filtration is, it does not remove dirt from the water. Your tank is a closed system and the only way you can get dirt and waste out is to change your water. Shrimp and snails provide limited mechanical filtration, in that they like to eat waste food, but they still poop out what they eat and add it back to the ecosystem.
Chemical filtration means carbon. In the same way that you can buy a brita pitcher to make your tap water taste better, you can put a carbon pad in your tank to absorb some impurities. Pet stores will tell you that carbon is the best thing ever. Pet stores tell you that because your carbon will run out and you'll have to spend more money on carbon every month. If you have medicated your tank, or if you have driftwood in your tank that's staining your water brown, carbon is useful to have. Otherwise, I recommend you don't have carbon in your tank. When you throw away your carbon pad, you'll also be throwing away a lot of your biological bacteria colony, which is bad.
Despite what some people tell you, a full carbon pad does not leak toxins into the water. It drives me absolutely nuts when people say that and I will go fuckin' science on your ass if you demand to know why.
Should I filter my tank?
Be honest--How on top of water changes are you? An unfiltered tank can get toxic or lethal levels of ammonia very quickly without changes. If your tank is 1 gallon, I suggest changing the water twice a week (once every 4-5 days, really). If your tank is 2.5 gallons, you need to change it every single week. You can't miss a week, and you really shouldn't miss a couple days. You need to be on top of your water changes every single week. If you travel a lot, work intense hours, get sick, stay at your friend's house a lot, and might miss a water change, consider getting a larger tank with a filter.
Is your tank 2.5 gallons? If it is, you can go either way on the filter. However, due to the smaller water level, your biological filter is more likely to "crash" or "hiccup," which means some of your bacteria will die and stop filtering the water. If that happens, nitrite can build up in your tank. Nitrite is more toxic to your fish than ammonia. If you have a filtered 2.5 gallon tank, be ready to check your water quality frequently.
Is your tank too big to change all the water at once? If it's going to be difficult for you to change all the water, get a filter. With a working biological filter, you only need to change 15-25% of the water every week.
What kind of filter should I get?
Remember that the most important kind of filtration for betta tanks is biological filtration. To grow your biological filter colony, you need porous places for the bacteria to live, and a source of oxygen for those bacteria. The bacteria don't float freely in your tank water; they grow on surfaces like your filter media, your gravel, and your cute pink castle your betta swims through. Most grow on the filter media, because the point of a filter is to make a good habitat for your bacterial colony.
My favorite filter for betta fish are sponge filters.
A sponge filter is a piece of sponge that is built so you can hook an air tube to it. You use an air pump to make bubbles in the sponge, and the bubbles rising out of the exit tube creates suction that draws new water through the sponge. The bacteria grow on the sponge, and the sponge provides some limited mechanical filtration. Sponge filters make very small currents (just the bubbles from the exit tube), which are good for betta fish. They're cheap too, but you can't buy them at pet stores. I get mine on Amazon. If you have shrimp in your tank, be ready for them to spend a lot of time eating off of the filter sponge.
For larger tanks, I'm a fan of undergravel filters.
Undergravel filters aren't super popular in the fishblr community, but I have never had a problem with mine as long as I clean the gravel properly.
An undergravel filter works similarly to a sponge filter. You use rising bubbles to pull water through your gravel on the bottom of your tank. The bacteria grow on the gravel.
There are two kinds of undergravel filters that I've seen. What I'm going to call "real" undergravel filters have a grating that covers the entire floor of your tank, and two lift tubes/bubble tubes (one at each end). A lot of cheap tanks now have what I call "fake" or "nonfunctional" undergravel filters, which have a very small grating that covers maybe 25% of your tank floor, and has one lift tube (usually in the center). The fake undergravel filters don't get enough lift from the central tube, and don't pull enough oxygen through the gravel to support good colonies. If your tank came with one of these, I suggest you throw it out and get a sponge filter.
If you want a filter that you can buy at the pet store, I suggest modifying a HOB or hang on back filter.
These are cheap and maybe came with your tank. It's got a tube that goes into your tank, pulls water into the filter, and returns the water to the tank via a little waterfall-looking thing.
Most HOB filters come with a carbon pad, which isn't a great home for bacteria. However, if you go into the canister filter section of the filter aisle, you'll find all sorts of great biological media. You can get sponge, ceramic noodles, bioballs, whatever they're selling that has good bacteria growing properties. Then just shove whatever it is you bought into the empty space in your HOB where your carbon used to be.
If you can find a good one, I suggest getting a nice prefilter sponge. This is a sponge you can put over your intake tube on your HOB. It reduces the current for your betta and provides even more places for bacteria to colonize. If you have a sponge with a lot of surface for bacteria, you don't need to put anything in your HOB, and use the sponge for all the filtration.
Some HOB filters have too high a water current coming from the water return waterfall, and you will need to baffle it. A lot of people use water bottles (google for a tutorial) and I've had good luck putting filter floss in the output.
If you're limited to the filters Petsmart sells, I think the HOB is the best option.
Internal filters can work, and your tank kit might have come with one. These work like a HOB and either use a propeller or rising air bubbles to draw water into it, and then return the water through a little chute. I don't like them because I feel like they take up a lot of space in the tank (compared to a sponge or HOB), the water flow is usually difficult to control, and they're built to take carbon. Some of them have electronics that remain submersed in their plastic housing, and I worry what'll happen to the fish if that leaks and gets water on the power supply.
If your tank kit came with an undergravel or an air-powered internal filter, the easiest thing to do is to buy a sponge filter, because you've already got the air pump and the tubing.
There's other kinds of filters, like canisters and fluidized sand, but they're expensive and usually overkill for a small betta tank.
Up next I guess I'll just ramble about how to decorate a betta tank for ten pages and then pretend I wrote something deep!