Index
Find the thing you are actually looking at — the haze, the smell, the reading that will not move — and start there. Every page below is written the same way: symptom, what to test, what usually causes it, and what to do first.
16 problems · 2 to act on today · 10 this week · 4 to watch
None of these pages opens by selling you something. They open by asking what your test kit says, because a water change costs nothing and fixes a startling proportion of what brings people here. Where a fix genuinely needs a piece of equipment, it says so and links to the category — and where it needs patience instead, it says that.
Act this week
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.
4 causes · 3 tests first
Act this week
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.
4 causes · 3 tests first
Watch it
Yellow water that is otherwise perfectly clear is almost always dissolved rather than suspended — tannins leaching from driftwood or leaf litter, or accumulated organics from a tank that is overdue a change.
4 causes · 2 tests first
Act this week
A still, shiny film on the surface is biofilm and dissolved protein collecting where there is no agitation to break it up.
4 causes · 3 tests first
Act today
A tank under about eight weeks old with an ammonia reading is not broken — it is unfinished.
5 causes · 3 tests first
Act today
Zero ammonia with high nitrite means the first half of your biological filter is working and the second half has not caught up.
5 causes · 4 tests first
Act this week
If nitrate is back above 40 ppm within days of a change, dilution is not your lever — the input is.
5 causes · 2 tests first
Act this week
A moving pH is a KH story nine times out of ten: the water has run out of the carbonate buffer that holds it steady.
5 causes · 2 tests first
Watch it
A brown, dusty coating that wipes off easily and appears in a tank under three months old is a diatom bloom.
4 causes · 2 tests first
Watch it
Green spot algae are small, hard, firmly attached dots that will not wipe off with a cloth.
4 causes · 2 tests first
Act this week
Dark tufts that feel tough and grip hard are what most keepers call black beard algae.
5 causes · 2 tests first
Act this week
Flow falls for mechanical reasons almost every time: a blocked intake, media saturated with debris, or an impeller wrapped in something.
5 causes · 2 tests first
Watch it
A filter that changed its noise overnight is telling you about air, the impeller, or something touching something else.
4 causes · 2 tests first
Act this week
Take two readings, one early morning and one late evening, from a thermometer that sits in the water.
5 causes · 2 tests first
Act this week
Disagreement between two kits is a procedure question far more often than a product question.
5 causes · 4 tests first
Act this week
The four standard tests measure four things, and none of them is smell.
5 causes · 3 tests first
Start from what you can see. The table matches an appearance to the first thing to measure, which is usually enough to land you on the right page.
| What you see | Test this first | Start here |
|---|---|---|
| Milky white, appeared in a day or two | Ammonia and nitrite | The water has gone cloudy and white |
| Green, cannot see the back glass | Nitrate, then look at the light | The water has turned green and I cannot see through it |
| Clear but amber, like weak tea | Nitrate and KH | The water is clear but yellow, like weak tea |
| Shiny still film on the surface | Temperature, and look at the outflow | There is an oily film sitting on the surface |
| Brown dust over everything, new tank | Nitrate and ammonia | Everything is coated in brown dust |
| Water looks fine, room smells | Ammonia, then go looking physically | All four readings are fine but the tank smells bad |