It is hard, but refrain from overcrowding your tank. Overcrowding is defined as putting more than one inch of fish per one gallon of aquarium water.


Some people quibble with this old rule, saying it doesn’t take into account the weight of the fish, which is a more accurate read on how much waste load the fish contributes. That’s true and they are right, but in the interest of keeping things simple the inch per gallon rule is so easy to understand and easy to follow. The fish by weight rule is not. In general, stick with the one inch of fish per one gallon of aquarium to keep your tank healthy.

The one way you can get around the one gallon of tank water to one inch of fish length is by increasing the filtration. If you have already overstocked your tank and have no way or reducing the fish load (ie, you do not want to set up a larger tank, or you do not know anyone who would take a few of your fish), then the next best thing may be to increase the filtration.

You do not need to throw out your old filter. What you are going to do instead is to get a second filter. It will need to be fairly powerful, with mechanical and biological filtration, and it should have a design that will not disrupt your tank set up too much. So underground filters are out. Power filters that hang on the side of the tank would work. And a canister filter would work, though unless your tank is over 50 gallons, adding a canister filter is probably overkill.

I recommend adding a power filter that is built for the same size tank as you have. Upgrading to a filter built for a tank larger than yours is not a totally bad idea, but because this site is focused on betta fish, and because the higher-powered power filters can create quite a lot of water currents, and bettas do not handle strong currents well, I would lean toward the most “peaceful” power filter you can get that will still substantially improve the water quality to make up for the tank having too many fish.

This is a walk-through of the basic techniques around aquarium filtration and how they apply to betta tanks.


This information also applies to community tanks of all kinds, but is more focused on freshwater than saltwater tanks.

Filtration is optional for a single betta bowl

If you have just one betta in a bowl or aquarium, you don’t need to get a filter. Personally, I don’t like the noise a filter adds, and one of the finest things about keeping a betta a bowl without a filter is the silence. If you skip the filter, though, you will need to stay on top of water changes. A two gallon bowl should have a complete water change every 7-10 days. Five gallon aquariums should have 50% of the water changed once a week.

When to get a filtration system

If you’ve got a community tank, you ought to get a filter. Also, if you’ve got a lot of plants in your aquarium, you may need to get a filter to keep up with the inevitable leaf shedding and decomposing that goes along with having plants.

Having a filtration system does not mean you can skip doing water changes. You should still be removing 20% of the water from the tank every week and replacing it with treated tap water.

Also, filters need to be cleaned out every week or so. If you do not thoroughly rinse out the filter sponges and/on “filter media” your filter pump will have to work much harder and you will shorten your filter’s lifespan by 50% or more. A good-quality filter, treated well, can last ten years or more. One that’s neglected may break in 9 months.

Types of filtration

There are three kinds of filtration: biological, mechanical and chemical.

Biological filtration works by making use of the “good” or nitrifying bacteria that break down fish wastes into nitrate and nitrite. Biological filtration works with (and basically, is) the nitrogen cycle you learned about in the section on cycling your tank. Biological filtration is used in just about every filter available right now. It requires a lot of surface area, so sponges and other porous materials are commonly used to maximize the bacterial action.

Mechanical filtration means actually pulling bits of waste and whatnot out of the water. Usually this is done by running water through a sponge. Actually, the sponge is performing both mechanical filtration by grabbing the particles, and biological filtration as the little bacteria that live in all the crevices of the sponge break down the captured particles.

Chemical filtration means the filter is removing dissolved compounds from the water. Most fish filters focus on biological and mechanical filtration; chemical filtration is used only sparingly. You are kind of using chemical filtration when you treat tap water with water conditioner to get the bad gases and heavy metals out.

The only time you’d really need chemical filtration after water treatment is if you wanted to add a bag of activated carbon to your filter to absorb odors, or if you had an ammonia problem and wanted to put a bag of “ammo rocks” or “ammo-lock” in your filter. There are also nitrate and nitrite absorbing “rocks” or granules that you can put into a plastic mesh bag and then put into your filter.

If you have enough space in your betta tank (like more than five gallons – ten is better), its pretty natural to want to add some more fish. While bettas are territorial, and they may fight with any fish you put into their tank, some fish make much better (betta?) companions for bettas than others.

Here are some good choices for betta tanks. Remember… you really ought to have at least a ten gallon tank to give your betta some room, and twenty gallons is better.

1) Cherry barbs

2) Rasboras. There are many kinds of rasboras. Harlequin, scissortail, red-tailed, clown and pygmy rasboras are all good choices for a betta tank. These fish all school (except for the clown rasboras), so you should get at least six of them to keep them happy. Rasboras won’t grow larger than two inches, max, and many stay smaller than that. They are also colorful, so they’ll complement your betta’s looks. A little band of schooling fish is fun to watch and it makes a nice counterpoint to a lone betta.

3) Danios

4) Small barbs

5) Angel fish

6) Cory catfish

7) Coolie loaches

Glass catfish

This is just a starter list. Here’s how to tell if a fish would be a good companion for your betta.

Swimming habits – Pick fish that have similar swimming habits to your betta. In other words, fish that streak around near the surface of the water are going to cause problems – fish like sharks (no, not ocean-going sharks… freshwater fish called “sharks”) and hatchet fish.

Temperature needs – Choose fish that like to be kept a bit warmer than the average community tank. If your betta is happiest at 80 degrees (or at least 78 degrees), choose fish that will be happy at that temperature. Goldfish, for example, are cold water fish and probably aren’t good companions for bettas. Either your betta is going to be cold all the time, or your goldfish is going to be hot all the time.

pH needs – Bettas are pretty easy to accommodate when it comes to pH. Most standard community fishes won’t give you a problem here, but be sure to check. Discus fish, for example, like the gentle currents and warm temperatures that bettas do, so they seem like good tank mates until you consider pH. But discus like a pH around 6.5 or lower, and that’s going to be hard on your betta.