It’s been said that fish tank owners are not just keepers of fish but also keepers of water…
Our aim is to keep our tank water clean and well filtered and create the best environment for our fish.
But just because the water looks clean doesn’t mean it’s necessarily completely healthy for our fish tank inhabitants…
Ammonia and nitrite in the water are toxic for fish and should be tested regularly, particularly with new fish tanks.
Similarly, nitrate and phosphate, whilst not harmful to our fish (unless in very high concentrations), need to be monitored as high levels can result in algae problems.
Alkalinity and hardness tests may also be required if you have special requirements such as a specialist planted tank or fish that require very hard or soft water.
Let’s have a look at what you should be testing for:-
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What to Test For In a Freshwater Fish Tank
You should incorporate regular testing of your tank water in your maintenance schedule.
In some cases, your local fish store may perform this service (for a fee) but we’d recommend getting a good test kit and doing it yourself.
If you have a problem you may need to perform a test quickly and get immediate results so you can take action.
At the basic level this is what you should be testing for:-
Test the Temperature
Ok, this is pretty obvious if you’re keeping tropical fish but you should have a thermometer measuring your tank’s temperature at all times.
A simple glass one in the tank will suffice or you may prefer a digital reader with a measuring probe.
The safest way to keep your tank temperature stable if by using a temperature controller to control your aquarium heater and alert you if the temperature gets too hot or cold.
Test For Ammonia
Ammonia arises from fish waste, uneaten food, and dead or decaying matter.
It is particularly an issue when starting a new fish tank as it goes through the nitrogen cycle and should be tested every few days until it is zero.
It can also arise in established tanks if the tank is overstocked, through overfeeding, the filters are not regularly cleaned or the water is not changed periodically.
We recommend testing for ammonia on a weekly basis or anytime a fish dies or if fish appear sick or distressed.
Test For Nitrite
Again, Nitrite will spike as part of the nitrogen cycle on new fish tanks and should be tested every few days until it is zero.
It can also arise in established tanks and should be tested for at least monthly or anytime a fish dies or if fish appear sick or distressed.
Excess levels of Ammonia and Nitrite in established tanks can be addressed by doing an immediate 20-25% water change and then adding a bacteria supplement to the tank.
Our article, Best Fish Tank Bacteria Supplements provides more guidance and recommendations.
Test For Nitrate
Nitrate is the end product of the nitrogen cycle and is not as toxic as ammonia or nitrite to fish but it must not be allowed to reach very high levels.
High nitrate can also lead to algae problems so should be tested for at least monthly.
Regular 10-25% water changes will keep nitrates in check provided your tank is not overstocked and you are not overfeeding.
Test The PH
PH is the measure of the acidity of the water. The lower the PH the more acidic the water is.
It’s important to know and test the PH level of your tank water as sudden changes can result in fish distress or even loss.
Some fish and plants also require low or high PH water, so knowing the PH of your tank will help determine if you can keep them successfully.
You should test the PH of your tank at least monthly.
At a more advanced level, you should also consider testing for the following:-
Test The Alkalinity Level
Alkalinity is the capability of water to neutralize acids and is therefore a measure of the ability of water to keep its pH stable. In fish keeping this is usually called the buffering capability.
As fish tanks age, they slowly lose this buffering capability which could result in unwanted PH changes.
Buffering compounds can be purchased at your local fish store or online to increase the water alkalinity and stabilize the pH level.
Test The Phosphate Level
Phosphate could also be called ‘algae food’ so if you’re having problems with algae, high phosphate levels are the likely culprit.
This can arise from using low-quality dried foods or overfeeding.
Several types of filter media are available to help you reduce phosphates and again, they can be purchased at your local fish store or online.
How to Test Your Freshwater Fish Tank
The short answer is to get yourself a test kit.
There are many different ones available but, please, do yourself a favour and stay well clear of the ‘test strip’ variety.
They are, at best useless and, at worst, a liability.
They might, just about, show a reading for ammonia and nitrite but you’ll struggle to identify an accurate reading.
For nitrates, phosphates and PH test strips are useless.
Far better to get a kit that allows you to chemically test a sample of your water for the various parameters individually.
Our article, Best Test Kits for a Freshwater Fish Tank, will give you the guidance you need.
Test Your Freshwater Fish Tank FAQs
What is the most important test for a fish tank? – Ammonia and Nitrite are poisonous to your fish so you should always test for those regularly.
Excess Nitrate will lead to algae problems so test for that and if too high, perform a water change.
You should also regularly test the PH and temperature of the water to ensure they remain within range.
How often should you test your fish tank water? – For new tanks going through the Nitrogen Cycle you should test for Ammonia, Nitrite, and Nitrate every 2-3 days.
For established tanks, you should test every 1 – 2 weeks or base it around your water change schedule.
What is the best test kit for a freshwater aquarium? – There are many good test kits available but overall we rate the API Freshwater Master Test Kit as the top one.
You can learn more by checking out our article, Best Test Kits for a Freshwater Fish Tank.
Are aquarium test strips worth it? – The short answer is no. They look like a cheap and convenient solution but are both difficult to read and very inaccurate. You’ll probably be better off not testing anything.