Read this first
If fish are dying right now, the single most useful thing you can do is a 50% water change with dechlorinated, temperature-matched water, and then turn up the surface agitation.
That is not a diagnosis. It is a holding action, and it buys you time — because the three things most likely to be killing your fish are all diluted by fresh water and all made worse by waiting.
Do not add medication yet. Most fish that die in home aquariums are not sick. They are poisoned, suffocated, or shocked. Medication does nothing for any of those, and it stresses an animal that is already struggling — and some medications will finish off your filter bacteria and turn a bad day into a total loss.
Now work through the questions below in order. The order matters: it runs from most likely to least likely, in this country specifically.
Question 1: How old is the tank?
If the answer is "less than two months", you have almost certainly found your cause and you can stop reading here.
A new tank has no bacteria colony to process fish waste. Ammonia builds up and chemically burns the fish from the inside — gills first. This is called New Tank Syndrome and it is, by a wide margin, the most common way beginner fish die everywhere on earth.
What it looks like: fish gasping at the surface, red or inflamed gills, clamped fins, lethargy, sitting on the bottom, refusing food. Deaths spread over days, often one at a time. The water usually looks perfectly clear — this is the cruel part. Ammonia is invisible.
What to do right now: test ammonia and nitrite. If either reads above zero, do 50% water changes daily until they do not. Stop feeding for two or three days — no food means no new ammonia, and a fish will not starve in that time. Then read How to Cycle a New Aquarium and understand that the tank is not finished yet.
If nitrite specifically is the number that will not come down, salt gives the fish chemical protection while the colony catches up — this is the one job salt is genuinely good at: the dose, and what else it does and does not do.
If you do not own a test kit, buy one today. You cannot fix what you cannot see, and every question below this one also depends on numbers. A liquid kit, not strips.
Question 2: Are they at the surface, gasping?
If several fish are at the surface at once, breathing hard, with no spots, no fungus, no visible marks — and it is worse in the early morning — this is oxygen, not disease.
It is the second most common killer here and it is seasonal. Warm water physically holds less dissolved oxygen than cool water, while the heat simultaneously speeds up the fish's metabolism so it needs more. In a UAE summer that gap opens up every afternoon, and it is at its worst just before dawn, when the plants have been consuming oxygen all night alongside the fish and the bacteria.
What to do right now: angle the filter outlet up so it breaks the surface. Turn on an air stone. Check the temperature. Do not medicate — there is nothing to treat.
The full version of this question — including the dawn test that separates oxygen from ammonia in two minutes, the dissolved-oxygen figures by temperature, and the four popular fixes that make it worse: Why are my fish gasping at the surface? The temperature side, including why ice cubes backfire: How to Cool an Aquarium in a UAE Summer.
Question 3: Did this start within a day of a water change?
If fish died within hours of you doing something helpful, then the something helpful is the suspect. Three candidates, all local:
- You forgot the dechlorinator, or used one that does not handle chloramine. Standing water overnight removes free chlorine but does nothing to chloramine, and you cannot tell which your supply has this week. This also kills your filter bacteria, so it causes an ammonia spike a few days later — a second wave of deaths that looks like a new problem.
- Temperature shock. In July, the "cold" tap in a building with rooftop storage can run at 35 °C or beyond. Pour that into a 28 °C tank and you have shocked every fish in it. Cold-side shock happens in winter for the same reason in reverse.
- You used hot tap water to mix the temperature. Water from the hot line has been sitting against a heating element and plumbing and can carry dissolved metals like copper — quietly lethal to shrimp, hard on fish.
All three, in detail: Is Dubai Tap Water Safe for Fish?
Question 4: Did you just add new fish?
Two different failures hide here, and they have different timelines.
The new fish died, the old ones are fine. Usually acclimation, transport stress, or a species mismatch — the fish wanted different water than you have. See How to Acclimate Aquarium Fish After Delivery.
The old fish started dying a week or two after the new ones arrived. That is disease, brought in with the new stock — and it is the one scenario in this article where medication is genuinely the answer. It is also entirely preventable with a quarantine tank, which is the single most-skipped and most-valuable piece of equipment in the hobby.
Or: you added too many at once. The bacteria colony is sized to the current bioload. Double the fish overnight and the colony cannot keep up — you get an ammonia spike in an established tank, which surprises people who assume cycling is a one-time event. Stock in small groups, a fortnight apart: How many fish can I put in my tank?
Question 5: Is it one fish, or several?
This single question separates two completely different problems, and getting it backwards wastes days.
| Likely cause | What to do | |
|---|---|---|
| One fish, others fine | Individual illness, injury, bullying, old age, or a fish that was already weak when you bought it. | Isolate it if you can. Watch the others. Do not dose the whole tank for one animal. |
| Several at once | The water. Ammonia, nitrite, oxygen, temperature, or something that went in the tank. | Test. Water change. Do not medicate. |
| One at a time, over weeks | Chronic stress — wrong water for the species, bullying, or nitrate quietly climbing. Or a slow contagion. | Test nitrate. Reconsider your stocking. Look at who is chasing whom. |
Question 6: Did anything go into the room?
This one catches people who have done everything else right, and it is worth thinking about seriously because the tank is not a sealed system — it is a large surface of water breathing your room's air, with a pump pulling that air through it.
- Aerosols. Insecticide, air freshener, hairspray, cleaning spray, perfume. Anything sprayed in the room can end up in the water. Pest control treatment in a flat has emptied entire tanks.
- Hands. Soap, hand sanitiser, lotion. Rinse thoroughly before your arm goes in.
- Paint, solvent, new furniture off-gassing in a closed, air-conditioned room.
- Copper. From hot tap water, from some medications, from certain plant fertilisers. Fish tolerate a little; shrimp die. If your shrimp died and your fish are fine, look here first.
If you suspect any of this: large water change, run activated carbon, and increase aeration.
Question 7: Did you clean the filter?
If deaths started two to five days after a thorough filter clean, you know what happened.
The bacteria you depend on live in the filter media. Rinsing that media under a tap kills them with chlorine. Replacing all the media at once throws them away. Either way the tank is effectively uncycled again, and the ammonia arrives a few days later — long enough that people rarely connect it to the cleaning.
The rule: rinse media gently in a bucket of old tank water, never under the tap. Never replace all of it at once. Filtration basics.
And the summer version: after a power cut, a filter that has been off for hours has gone anaerobic. Rinse the media before restarting, or you push a slug of foul water into the tank.
Question 8: Is the fish in the wrong water in the first place?
The slowest killer, and the hardest to see, because nothing dramatic ever happens. The fish just never thrives, and then one day it dies, and you blame the last thing you did.
UAE tap water is hard and alkaline — commonly around pH 7.5–8.2. A soft-water fish in it is under permanent low-grade stress: dull colour, no growth, susceptible to everything, short life.
The classic cases here: neon tetras (weak farmed stock, sold to beginners with new tanks), goldfish (a cold-water fish at 29 °C all year), discus (a genuine soft-water specialist), Caridina shrimp (need soft water — while Neocaridina need your hard water; they look similar in a shop tank).
No medication fixes a species mismatch. The fix is different fish: what actually survives here.
The symptom table
Use this to confirm, not to diagnose. The questions above are the diagnosis.
| What you see | Most likely | Not |
|---|---|---|
| Gasping at the surface, several fish, clear water | Oxygen, or ammonia | Not a disease |
| Red or inflamed gills, clamped fins, lethargy | Ammonia burn | Not a bacterial infection |
| White spots like grains of salt | Ich — often triggered by a temperature swing | Real, and treatable. Fix the temperature stability too. |
| Died overnight, no symptoms, all at once | Something entered the water: chlorine, aerosol, copper, or a big temperature swing | Not a disease — diseases take days |
| Faded colour, no growth, dies eventually | Chronic stress: wrong water, nitrate, bullying | Not something to medicate |
| Shrimp dead, fish fine | Copper, or soft water for a hard-water species | — |
| New fish died, established fish fine | Acclimation or transport stress | Not your tank, probably |
| Established fish dying a week after new arrivals | Disease brought in with the new stock | This one, medicate. |
The four numbers to have before you ask anyone for help
If you post in a forum or walk into a shop without these, nobody can help you and everyone will guess:
- Ammonia — should be 0
- Nitrite — should be 0
- Nitrate — should be low; if it is high, you are not changing enough water
- Temperature — and whether it is stable, which matters more than the number itself
Plus: how old is the tank, how many fish, and what changed in the last week. That is the whole diagnostic. Everything in this article is just those seven facts, read in the right order.
Common questions
My water is crystal clear, so it cannot be the water — right? Wrong, and this is the single most expensive misconception in the hobby. Ammonia, nitrite and low oxygen are all completely invisible. Clear water tells you nothing. A cloudy tank can be perfectly healthy; a spotless one can be lethal.
Should I use a broad-spectrum medication just in case? No. If the cause is water quality — and it usually is — medication adds a second stressor to a fish that is already struggling, and several common treatments damage the filter bacteria. That turns a survivable problem into a crash. Diagnose first. The same logic applies to reflexive salting: salt protects against nitrite and does very little else.
The shop says my fish had a parasite. Maybe. But if several fish died at once in a tank under two months old, it was ammonia, and the parasite is a story that fits the evidence less well than the test kit does.
How long should I wait before adding replacement fish? Until you know what killed the last ones. Restocking into an unfixed tank is just buying more fish to kill — and the second round is more expensive than the test kit that would have prevented it.
Everything reads zero and they are still dying. Then look at oxygen (agitation), temperature stability (not the number — the swing), and what went into the room. Those three are invisible to a standard test kit, and between them they account for most of the cases that look inexplicable.