There are several variables that affect how often you should change your betta’s water and the only true way to tell is to test your water with the appropriate aquarium test kits.

We change our betta’s water mainly for the reason that toxins accumulate over time that are extremely harmful to aquarium fish. These toxins are mainly ammonia, nitrite and nitrate, and are the byproducts of fish waste and decaying debris like food and dying plants. Ammonia and nitrite are the most dangerous and any accumulation of these toxins can lead to illness or even death. Nitrate, a waste product of bacteria in the water, is the least dangerous but can become toxic at high concentrations. To understand more about these compounds, visit All About Water.

Now that we know WHY we change our betta’s water let’s look at the variables that affect how quickly those toxins accumulate.

Tank Size:

Toxins will reach dangerous levels much faster in a small volume of water then they will in a larger one. This is just simple dilution. Imagine you have a dropper full of food coloring. A few drops in a bathtub full of water would hardly be noticeable at all, but add the same number of drops to a small glass and it would be quite noticeable. This is one of the main reasons that larger tanks are recommended for beginners while smaller ones are often considered to be more appropriate for advanced hobbyists. Many folks new to the hobby don’t realize this and think it will be easier for them to start with a small tank.


Photo by Clay Mask

The Nitrogen Cycle is a naturally occurring process where beneficial bacteria grow and reproduce to consume the toxins produced by fish and decaying matter. This process has occurred all over the world in lakes, ponds, streams and oceans and will also occur in your aquarium if given the chance. Experienced aquarists usually monitor this process when they first set up their tanks. The cycling process itself can be quite stressful to fish but once it’s complete it creates a safer and more stable environment. An uncycled tank doesn’t yet have the bacteria in place to consume toxins and frequent water changes may be necessary to keep ammonia from building up. If you plan to cycle your tank you will have to stop performing full water changes. Once the process is completed, partial 25% water changes are all that is necessary to keep toxins at bay under most circumstances. To learn more about tank cycling visit Tank Cycling: The Fishless Method. Let’s assume, for now, that your tank is not cycled…

Fish Waste:

Fish waste is the main contributor to toxins in the water. Ammonia, which is severely toxic at even minute levels, builds as fish defecate and urinate. The rate at which these levels raise depend on the fish. Like humans, fish metabolize their food at different rates based on age, size and genetics. Water temperature may also affect how quickly fish produce waste. A betta’s metabolism will be faster in warmer water then it will be in cooler water.

Decaying Matter:

Uneaten food is another major contributor to ammonia build up. Food left to breakdown in the tank will quickly add to the toxicity levels. To avoid this, try not to overfeed your fish and remove any uneaten food within about 10 minutes of feeding. Also, the type of foods you feed may contribute to toxin build-up differently.


Plants can work for you or against you. Healthy plants will utilize some of the toxins produced in the tank and though they can’t replace water changes, they can help reduce the frequency you need to change your water. On the contrary, however, dead or dying plants will breakdown producing more deadly ammonia. Learn the needs of the plants you buy and remove any dead leaves to get the most out of your planted aquarium.


The quantity, size and species of the fish you keep determine your bioload, or the amount of “life” you can safely support in a given size tank. It’s difficult to quantify but a common calculation is one inch of fish per one U.S. gallon of water. If a grown female betta is 2.5 inches in length approximately, and you wanted 4 of them then you would need a minimum of 10 gal to support the bioload. (2.5 in x 4 fish = 10 gal) In reality there are so many other things at play, like the physical mass of the fish, their behavior and how they eat. For instance, you would never keep four male bettas in a 10 gallon tank because they are too aggressive and you would never keep four, 2.5 inch goldfish in a 10 gallon tank because they produce significantly more waste then your average hobby fish. Basically, for your purposes, remember the more fish you have in your aquarium, the quicker toxins will build and the more frequently you will need to perform your water changes.


The type of filtering system you have in your tank and how it is maintained will also affect your toxin levels. A suitable power filter with charcoal media may remove more waste then a simpler sponge filter.


Toxins may accumulate at a different rate depending on the type of substrate you use in your tank. A tank with a deep sandy bottom may hold more decaying matter then one with large gravel. Larger substrate may be cleaned easier with a siphon then smaller particles. A bare bottom tank, though less attractive, is often the easiest to clean.

What does all this mean?

Photo by Irina Souiki

Ok, now you know there is no simple answer as to how often you should clean your betta tank. Still, you need some direction. Fortunately, there is a good way to determine your individual cleaning schedule. Let’s assume, again, that your tank is uncycled and you will be performing regular full water changes. To determine how often you need to change your water start by doing a complete 100% water change, essentially starting from scratch. Treat the water with a good dechlorinator like Kordon’s AmQuel+ and NovAqua. Acclimate your betta and begin testing your water for ammonia starting that day. Be sure you are using a good aquarium test kit. Ammonia kits that come with a reagent bottle (or two) and a test tube are far more reliable then the dry dip-stick test kits. Be sure to purchase a “Salicylate” test kit rather then a “Nessler” test kit. In a Salicylate kit the color comparison chart is often green while the Nessler kit measures in shades of amber.

The first day you measure the ammonia levels should read 0. Continue to measure each day until trace amounts of ammonia become visible. As soon as you see ammonia you know exactly the maximum number of days you can go between water changes. If it takes 6 days for ammonia to present itself you should be doing full water changes every 5 days to avoid any measurable ammonia build-up. If it takes 10 days for your test to show ammonia, then you should do that water change by the 9th day at the latest. If you make any significant changes to your set up (I.e. add a new fish or significantly change the diet) repeat the test to be sure your water changing regimen doesn’t need a change itself.

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