Freshwater, water-first

Your tank went wrong. Start with what the water is doing.

Cloudy, green, smelly, warm, or a reading that will not move — almost all of it comes back to a handful of numbers and a bucket. Every page here opens the same way: what to test, what usually causes it, and what to do first. Almost none of it starts with a bottle.

Find the problem Start with the cycle

16 problems · 6 parameters · 19 maintenance tasks · 26 glossary terms

The order of operations

Test ammonia and nitrite. In a tank under two months old these two are the whole picture.
Change water. It is the only step that removes a problem rather than converting or masking it.
Look for the physical cause: something dead, a blocked intake, a light left on, a portion too large.
Only then consider a product, and only for the job it actually does.

If a fish looks unwell, test the water now — bad water is the most common cause and the one you can fix in an hour — and then contact an aquatic veterinarian. This site does not diagnose or treat fish.

Have readings in front of you?

Enter the four numbers your kit gave you and say how old the tank is. What comes back is what that combination usually means and an ordered list of what to do, starting with testing and a water change rather than with a product.

Enter what your kit says

Pick the nearest value on each row. The options are the readings a hobby kit actually resolves, so there is no way to enter a number your test tube cannot show you.

Ammonia (ppm)

Nitrite (ppm)

Nitrate (ppm nitrate ion)

pH (pH units)

Tank age

Reading

Ammonia
0
In range
Nitrite
0
In range
Nitrate
20
Borderline
pH
7.2
In range

This is what a healthy tank looks like partway through its week: nothing at the front of the chain, and the end product accumulating as it should.

Do these in order

Change 20 to 25% of the water on your normal day and vacuum a section of substrate.
Record the pre-change nitrate reading — that number is what tells you whether the interval is right.
No further action. Nothing here calls for a product.

Before anything else

If any fish is gasping at the surface, sitting on the bottom, refusing food, or showing marks or unusual coloration, change water now and then contact an aquatic veterinarian. This tool reads water. It does not diagnose animals, and nothing sold on this site should be used as a treatment.

Start here

All Fixes

Act this week
The water has gone cloudy and white
A white or gray haze that appeared over a day or two in a tank under two months old is almost always a bacterial bloom, and it clears on its own once the biological filter catches up.
Test first: Ammonia, Nitrite, Nitrate
Act this week
The water has turned green and I cannot see through it
Green water is a bloom of single-celled algae suspended in the water column, and it needs two things at once: light and dissolved nutrients.
Test first: Nitrate, Ammonia, pH
Act today
My new tank is reading ammonia and there are already fish in it
A tank under about eight weeks old with an ammonia reading is not broken — it is unfinished.
Test first: Ammonia, Nitrite, pH
Act this week
Nitrate is high no matter how often I change water
If nitrate is back above 40 ppm within days of a change, dilution is not your lever — the input is.
Test first: Nitrate, Ammonia
Watch it
Everything is coated in brown dust
A brown, dusty coating that wipes off easily and appears in a tank under three months old is a diatom bloom.
Test first: Nitrate, Ammonia
Act this week
The filter output has slowed to a trickle
Flow falls for mechanical reasons almost every time: a blocked intake, media saturated with debris, or an impeller wrapped in something.
Test first: Ammonia, Nitrite

The full index is grouped by what you are looking at: clarity and color (4), cycle and parameters (4), algae (3), equipment (3), testing (2). 2 of the 16 are marked act today, and those are the ones where a water change tonight matters.

The six numbers

All Parameters

ppm total ammonia nitrogen
Ammonia
Total ammonia — the ammonium ion plus free ammonia, added together.
ppm nitrite
Nitrite
Nitrite ion, the intermediate product between ammonia and nitrate.
ppm nitrate ion
Nitrate
The nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number.
pH units, with KH in dKH
pH and KH
Acidity of the water, plus the carbonate buffer that resists changing it.
°F, with °C alongside
Temperature
The temperature of the water itself — not of the room the tank sits in.
ppm as Cl₂
Chlorine and Chloramine
Residual disinfectant carried in from the tap.

Half of it comes back to the cycle

Cloudy water in a new tank, nitrite that will not fall, an ammonia reading two days after a filter clean — these are the same story told from different points in it. The pillar page walks through all 9 parts of it: what is actually being built, what the test kit shows at each stage, and the one habit that undoes the whole thing.

Read the cycle page

What the kit reads at each stage
Stage	Ammonia	Nitrite	Nitrate
Stage one — ammonia climbs	Rising, often 1 – 4 ppm	0 ppm	0 ppm
Stage two — nitrite appears	Falling toward 0 ppm	Rising, often 1 – 5 ppm	Just detectable
Stage three — nitrate only	0 ppm	Falling to 0 ppm	Climbing steadily

Maintenance

Most problems are an interval that slipped

19 tasks across five intervals, each with the reason it exists and what legitimately moves it. Nothing on that list is urgent on the day it is missed, which is exactly why it gets missed.

See the schedule

Work out the volume

A tank sold as twenty gallons does not hold twenty gallons

Measure the glass, allow for what the substrate takes up, and find out what a 25% change is actually moving — in gallons, in liters and in buckets. Both formulas are printed on the page, along with what they do not account for.

Open the calculator

Equipment, and what it cannot do

All Gear

6 categories and 33 products, described from their retailer listings. No prices, no ratings, and no claim that anything here was tested. One category measures the room rather than the tank, and says so on every card.

Water Conditioners & Additives
making tap water safe to put in a tank
Filters
turning water over and housing the bacteria
Filter Media & Cartridges
what actually does the filtering, and when it gets replaced
Test Kits
knowing the number instead of guessing it
Room Temperature & Humidity Monitors
watching the room the tank lives in
Decorative Stones & Pebbles
looks, with the caveats stated plainly

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What this site will not do

Diagnose or treat a sick fish. Symptoms in an animal are a reason to test the water immediately and then to call an aquatic veterinarian. There are no disease names and no treatment protocols anywhere here.
Publish a dose for anything, including ordinary conditioners. Concentrations differ enormously between products that look alike, and the only correct figure is on the bottle in your hand.
Tell you to replace all your filter media at once or rinse it under the tap. Both destroy the bacterial colony you spent weeks growing, and both are common advice elsewhere.
Offer a product as the first move on a maintenance problem. Testing and a water change come first, on every page.