Should I change water during bacterial bloom?

When starting a new aquarium, it’s common to experience a bacterial bloom within the first few weeks. This bloom occurs when there is an excess of nutrients in the water for the bacteria to feed on. As the bacteria rapidly multiply, the water becomes cloudy and sometimes smelly. While unsightly, bacterial blooms are harmless and a sign of a normal cycle in establishing an aquarium’s ecosystem. However, many aquarists wonder if they should change the water during this period. Here’s a quick look at whether or not you should change water during a bacterial bloom.

Quick Answer

In most cases, you should not change the water during a bacterial bloom. Changing water can disrupt the bloom and delay the aquarium’s cycle. It’s best to let the bloom run its course. The bacteria will consume the excess nutrients and the water will clear on its own within a week or two.

Should I Do a Complete Water Change?

Doing a complete water change during a bacterial bloom is not recommended. Removing all of the water will take out the beneficial bacteria that are trying to establish themselves. This can essentially reset the aquarium cycle and prolong the bloom.

The bacteria in the bloom are essential for processing excess nutrients like ammonia and nitrites. By changing the water, you lower the bacterial population and allow nutrients to accumulate again. This gives even more food for bacteria when they start reproducing again, making the bloom last longer.

Will a Partial Water Change Help?

A partial water change of 25-30% during a bacterial bloom may help lower excess nutrients a little. However, it’s still better to avoid changing water if possible.

The main nutrients that bacteria feed on are ammonia and nitrites from fish waste, excess food and dead plant matter. These dissolve into the water, so a partial water change will only dilute them somewhat. The remaining nutrients will still allow bacteria to thrive and prolong the bloom.

Can I Vacuum the Gravel?

It’s best not to vacuum the gravel during a bacterial bloom. This can remove a significant portion of the beneficial bacteria living in the substrate.

Gravel vacuuming is an important part of aquarium maintenance. But during the initial bloom, you want as much bacteria present as possible to establish a healthy ecosystem. Wait until after the bloom clears to start vacuuming.

Will Changing the Filter Media Help?

Changing filter media like cartridges, sponges and bioballs can also disrupt the bacterial bloom. The filter media provides an ideal surface for colonies of nitrifying bacteria to grow.

Removing this media tosses out the bacteria as well. It’s better to simply rinse filter media in old tank water if it’s getting clogged, rather than replacing it completely during a bloom.

Should I Use Filter Media from an Established Aquarium?

Using filter media from an existing tank can help cycle a new aquarium faster. But it’s still best not to change any media during an active bacterial bloom.

The bloom is evidence that enough beneficial bacteria are present and multiplying already. Let these bacteria establish fully before swapping out media.

Will Purigen or Carbon Filtration Help?

Using chemical filtration like Purigen or activated carbon is also not recommended during a bacterial bloom. These can remove organic compounds that serve as food sources for the beneficial bacteria.

Reducing nutrients too much can starve out the establishing bacteria. This slows down cycling and allows excess nutrients to accumulate when bacteria levels drop.

Wait until after the bacterial bloom clears to start using Purigen or carbon. This allows bacteria populations to stabilize so they are not as impacted by removal of nutrients.

Should I Use Bacteria Supplements?

Using bacteria supplements can provide a boost of beneficial bacteria to speed up cycling. However, it’s typically not needed to clear up a bacterial bloom.

The bloom itself shows there are already enough bacteria present to process available ammonia and nitrites. Adding more bacteria via supplements won’t have any real effect.

Once the bloom starts clearing and bacteria levels drop, supplements can be used to replenish populations. But they won’t impact an existing bloom.

Are There Any Additives That Can Help?

The only additive that may help shorten a bacterial bloom is micronutrients like trace elements or fertilizers containing iron. These provide nutrients for the many types of bacteria involved in cycling.

However, only use a half dose or less during the bloom. Too many micronutrients can give bacteria more food and prolong the bloom.

Should I Stop Feeding Fish During the Bloom?

Reducing feeding somewhat can lower excess nutrients that bacteria use as food. However, don’t stop feeding fish completely during a bacterial bloom.

Fish still require nutrients from some daily feedings. Just feed a little less than normal, perhaps skipping a day of feeding per week. This modest reduction allows bacteria to process waste more easily.

Test water parameters like ammonia and nitrites to ensure they stay at safe levels for fish throughout the bloom. Do enough partial water changes to keep these in check if needed.

How Much Should I Feed?

A general guideline is to feed fish only what they can consume within 2-3 minutes, once or twice per day. This prevents uneaten food from fouling the water too much.

Pay attention to fish behavior at feeding time. Healthy fish will be eager to eat their full portion. Reduced appetite can signal that water quality is declining.

Can I Add Fish During the Bloom?

It’s best not to add any new fish while an aquarium is going through a bacterial bloom. The excess bacteria and nutrients can cause stress for fish.

New fish have not adapted to the tank conditions yet. Exposure to the bloom may weaken their immune systems and lead to illnesses.

Wait until after the water clears to introduce new fish. This gives time for water parameters to stabilize and provides a healthier environment.

What About Hardy Fish Like Danios?

Even hardy fish like danios, white clouds and platies are better off not being added during the bloom. Although they can tolerate poorer water conditions, the bloom can still tax their systems.

It’s safest to let the aquarium fully cycle first before adding any fish. The clearer, cleaner water will get new fish off to a healthy start.

When Will the Bloom Go Away?

Most bacterial blooms will clear within 1-2 weeks after initially appearing. The bacteria populations will reach a balance with the amount of nutrients available.

As the bloom declines, you’ll notice the water become clearer and any smell dissipate. Testing will also show ammonia and nitrites leveling off at safe ranges for fish.

The bacteria levels and water parameters may fluctuate for another few weeks after the bloom. But overall water quality and clarity will continue improving.

What If the Bloom Lasts Longer?

While most blooms last up to 2 weeks, some can persist longer if excess nutrients remain. Make sure you aren’t overfeeding fish or leaving decaying plant matter in the tank.

Test to ensure ammonia and nitrites are at safe levels. Do a partial water change if they start creeping up. The bloom should eventually clear as long as nutrients are controlled.


Having patience is key to dealing with bacterial blooms in a new aquarium. While unsightly, changing water, media and other conditions will usually just prolong the bloom.

Allow the bloom to run its course as bacteria establish to process waste and nutrients. Avoid changing filter media, gravel vacuuming and using chemical filtration during this period.

Moderate feeding to prevent excess nutrients. Test water parameters regularly and change enough water to keep fish safe. Once the bloom clears after a week or two, your aquarium will be well on its way to a healthy, stable cycle.

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