Fish Tank Filtering – Ph, NItrites and Nitrates

Most people start searching about fish tank filtering after a stressful test result: pH drifting, nitrite suddenly showing up, or nitrates creeping higher each week. In a closed glass box, these numbers aren’t trivia. They’re early warnings that the biology inside the filter is keeping up—or quietly falling behind.

Below is a clear, practical view of what pH, nitrite and nitrate actually mean in an aquarium, what “normal” looks like, and what to do when a reading is off. The focus is on steady water quality, not quick fixes.

Fish tank filtering: what the filter is really doing

A filter is less a “vacuum cleaner” and more a moving river. It keeps water flowing through surfaces where waste can be trapped, and—more importantly—where beneficial bacteria can live and convert toxic nitrogen waste into less harmful forms.^{1, 2}

The three types of filtration (and why one matters most)

• Mechanical filtration traps suspended debris (uneaten food, plant fragments, fine particles).¹
• Biological filtration houses bacteria that process fish waste (the engine room of a stable tank).^{1, 2}
• Chemical filtration uses media (such as activated carbon) to adsorb certain dissolved substances; it’s optional and situation-dependent, not the core of day‑to‑day stability.¹

The nitrogen cycle: where nitrite and nitrate come from

Fish excrete ammonia, and leftover food and decaying plant matter also break down into ammonia. In a cycled aquarium, bacteria convert ammonia → nitrite → nitrate.^{1, 2}

This is why a brand-new tank often has a dangerous “new tank” period: the bacteria colonies take time to establish, and ammonia or nitrite can rise before the system catches up.^{1, 2}

What “cycled” looks like in plain numbers

• Ammonia: ideally 0
• Nitrite: ideally 0²
• Nitrate: present to some degree in most stocked tanks, and controlled with maintenance (water changes, plants, sensible feeding).^{1, 2}

pH in aquariums: stability beats chasing a “perfect” number

pH is a measure of how acidic or alkaline the water is. What matters most for many common aquarium fish is a stable pH that suits the species, rather than frequent adjustments that cause swings.²

pH also changes how dangerous ammonia can be. As pH (and temperature) rise, a greater proportion of total ammonia can exist in a more toxic form, which means the same waste load can hit harder on a high‑pH day.^{3, 4}

Practical pH checks

• Test at roughly the same time of day each time, so you’re comparing like with like.
• If pH is drifting over weeks (not hours), check carbonate hardness/alkalinity as well—low buffering can let pH fall suddenly in established systems.⁵
• Avoid “pH up/down” corrections unless you understand the cause. Quick swings can stress fish more than a slightly imperfect but steady number.

Nitrite: small numbers, real risk

Nitrite is produced during the middle stage of the nitrogen cycle. In a stable, cycled tank it should read zero; any detectable nitrite suggests the biofilter is not keeping up or has been disrupted.^{2, 5}

Common reasons nitrite appears

• A newly set-up tank that hasn’t finished cycling.^{1, 2}
• Over-cleaning filter media, especially in tap water (chlorine/chloramine can damage beneficial bacteria).¹
• A sudden increase in feeding or stocking that outpaces the existing bacteria colonies.
• Filter stoppage or reduced flow (clogging, power interruption) that starves bacteria of oxygenated water.^{1, 5}

Nitrate: the slow build in the background

Nitrate is the end product of nitrification. It’s generally less acutely toxic than ammonia or nitrite, but it can accumulate in closed systems and become a chronic stressor at high levels.^{1, 2, 5}

In many freshwater community tanks, nitrate is managed with routine partial water changes and sensible feeding. Live plants can also use nitrate as a nutrient, acting like a quiet, steady drain on the system.^{1, 5}

Testing and monitoring: turning numbers into a routine

Test kits don’t prevent problems on their own. They simply tell you which part of the system is under strain—waste input, bacterial processing, or maintenance pace.^{1, 2}

How often to test

• New tanks (first 4–6 weeks): test frequently for ammonia and nitrite while the biofilter establishes.^{1, 2}
• Established tanks: test every 1–2 weeks, and any time fish behave unusually, you’ve cleaned the filter, or you’ve changed stocking/feeding.¹

If a reading is “off”, start with these calm, high-impact moves

• Stop overfeeding. Feed lightly until the numbers settle.
• Do partial water changes to dilute toxins, especially if ammonia or nitrite is detectable.^{1, 2}
• Check filter flow and aeration. Nitrifying bacteria need oxygenated water moving across their surfaces to function well.^{1, 5}
• Review anything that changed recently (new fish, new food, filter media replaced, medications used, power outage).

Choosing the right filter (without getting lost in specs)

Pick a filter that can move enough water for your tank size and provide plenty of surface area for biological media. The goal is stable biological filtration with steady flow, not just crystal-clear water.^{1, 2}

Match your setup to your bioload (how much waste the tank produces). A lightly stocked planted tank is a different world from a busy goldfish tank, even at the same volume.

Maintaining your filter for steady performance

Filter maintenance is where many tanks tip from stable to shaky. Clean enough to keep water moving, but gently enough to keep the bacterial colony intact.¹

A simple, safe maintenance rhythm

• When flow slows, rinse sponges/media in a bucket of tank water, not under the tap.¹
• Avoid replacing all media at once. If you must replace something, stagger changes so bacteria can repopulate.
• Keep the filter running continuously. Long stoppages can reduce oxygen and harm the biofilter community.⁵

Final thoughts

A healthy aquarium is usually a quiet one: steady pH, zero nitrite, and nitrate rising slowly enough that your weekly routine keeps it in check. When numbers move, treat it like field notes—evidence of where the system is under pressure—then respond with small, consistent corrections rather than sudden chemical swings.^{1, 2}

References

1. RSPCA Knowledgebase — Why is water quality important when setting up a fish aquarium?
2. RSPCA — Choosing an aquarium for pet fish (filters, cycling, safe ammonia/nitrite)
3. NSW Department of Primary Industries — Monitoring ammonia
4. University of Florida IFAS Extension — Ammonia in Aquatic Systems
5. Oklahoma State University Extension — Nitrification and maintenance (oxygen/pH considerations)