Fish are dying and I don't know why

The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.


Aquarium Advice Regular
Oct 4, 2023
Morning all,

Question for the braintrust ...

As I mentioned on my post last week, we just got a new 20 gallon tank and my kids are super excited to get into this hobby.

We were hoping to try a fish-in cycle but nothing seems to be going right.

Last week, we tried introducing 3 glofish tetras. All water levels (ammonia etc.) were at 0, pH approximately 7.8 and water temp around 25-26 C. I had added some seachem stability as well.

The water was very cloudy, which I assume was a bacterial bloom, but the fish store said that is should be OK to introduce the fish.

I acclimated them for 20-30 minutes and then added them to the tank. Not much later, one of them was already at the surface gasping for air. By the next morning, he was dead and stuck to the filter intake. The other two were alive but very lethargic. A few hours later they were gone as well.

Since then, I've been testing daily and adding stability almost every day. Number have been steady at 0 except for one day where the ammonia might have been between 0-0.25 ppm. Went back down though.

Yesterday I tried again with three glowlight danios (recommended by LFS). Water has been completely clear for a few days already and levels all at zero. I also added a small air stone to improve oxygenation just in case.

This morning, one of the danios was dead, again stuck to the filter. Another one died within a couple of hours and the last is gasping at the surface again. Tried feeding him a small pellet but he totally ignored it.

I'm at my wits end and don't know what else to do. My kids are understandably upset at seeing all of these dead fish

Any suggestions or recommendations would be greatly appreciated.
Reasons why fish might be dying in such a small timescale.

- Not using a water conditioner or not using enough. What water conditioner are you using?
- Not properly acclimating the fish. How are you acclimating?
- Lack of oxygen. Do you have an airstone in the tank or does your filter output create surface turbulance?
- You are buying fish that are already sick, and the shock/ stress of the move is the final straw. Where are you buying fish from? Are they reputable? When you go to the store are there dead or sick looking fish in the tank?

Another possibility. Does your tap water go through a water softener?

Just to save you some money. Stop using stability, it wont be doing anything useful. These bottled bacteria products usually don't do anything, and stability is one of the worst. If you want to add one of these products, Fritz #7, Fritz Turbostart, Dr Tims One and Only and Tetra Safestart are some of the better ones, but adding over and over wont really help much. They either work or dont.

Thanks very much for the prompt response.

To answer your questions:

1) Using Seachem Prime, based on the dosage recommendations. I've also been adding a bit (1-2 mL) every other day or so just in case.

2) Acclimating by putting the bag (from the store) floating in the tank for 20-30 minutes, adding some water from the tank into the bag and then scooping them out and placing them in the tank.

3) Yep. Have a small airstone (lots of tiny bubbles) and a 'waterfall' coming from the filter into the tank. Plenty of water movement and plenty of surface agitation.

4) Reputable LFS (not a 'big box' store). They are very knowledgeable and their fish all look healthy and well cared for.

5) No water softener.

I'm going to try and take a water sample to the store to get it tested there in case I'm doing it wrong, but I didn't see any hint of 'bad' colours in any of the testing I've done. Having said that, I only tested the pH once (when I first filled the tank) and it was at 7.8. Is that something that needs to be tested every day, and would that have such a negative impact on the fish?

Interesting comments about stability ... the LFS person highly recommended it, and I've read good reviews online (including from some folks who seem pretty knowledgeable) ... having said that, I have no way of knowing if it works, though. Given that I already have the bottle, I assume there's no harm in continuing to use it?

One thing I will add ... the fish I purchased yesterday were already somewhat stressed in the store, as they were doing a significant water change. However, the folks at the store thought they would be fine for the (50 minute) drive home and introduction to a new tank ... not sure if the excess stress may have killed them ... would it be so bad that they would all die in less than 24 hours??

Thanks again!
Nothing that a standard test kit will detect will kill fish in the short timescales you are seeing, unless you are talking about them getting very bad. Elevated ammonia, nitrite and nitrate will kill fish, but its usually over weeks.

Ammonia is more toxic at higher pH and temperature, so it will be more toxic in your aquarium than most other aquariums closer to a neutral pH and 24c. But to give you some comfort at a pH of 7.8 and a 26c temperature ammonia will start to give you long term problems at about 1ppm, and start killing fish overnight at 2ppm. If your pH crept up to 8, then 1ppm of ammonia will start to kill fish. So you do have to be careful to monitor things.

There are other things that can kill fish that an aquarium test kit doesnt detect though. Most water companies should have a comprehensive water report available online.

Ask the fish store what their water parameters are. If they are very different to your own you might need a more comprehensive acclimating process.

My local store injects O2 into the bag if the travel distance home is lengthy. Does yours do the same?

Stabilty shouldnt hurt anything, so may as well use up what you have. But ive never heard anyone have positive experiences of it on this forum (or any others im a member of for that matter), so in general i dont see it as having a good reputation. Seachem does do a very good job at marketing their products, and you always have to look at whether someone has a financial interest in pushing a specific product. Look for the affiliate links with online resources.

Seachem Prime is a good, cost effective water conditioner though. Its #1 for a reason. Some of their other products, not so much.
I found a water testing report from the city ... any specific parameters I should be looking at?

Chloramines are listed as 1.7 mg/L on average and avg. pH is 7.5. Hardness is 199 mg/L on average (calculated as CaCO3).

I checked the water again and it's actually just under 25, so not as high as I originally thought.

As far as the bag from the store, they seal it in a way that there's a huge bolus of air in the bag but I don't think they specifically inject O2.

What else should I try before introducing new fish? Very much want to avoid a third failure ...

Thanks again!
Im not really qualified to make a judgement on your water report beyond those parameters that are normally tested for. If you can post a link or screenshot, someone might understand it more than me. If there is a toxin it might not be from the tap, but may have come off something in your tank, which wouldnt be unheard of.

However, your water is treated with chloramine rather than chlorine. Chloramine is chemically bonded chlorine and ammonia, and its more stable than chlorine, and therefore more difficult to get rid of for fishkeepers. Seachem advise a 2x dose of prime for exceptionally high chloramine levels. Yours is say 1.7ppm which is higher than the legally allowed level of chloramine here in the UK, so try double dosing your prime, so say a capful for your 20 gallon tank.

A bit more on chloramine. Your water conditioner breaks the chemical bond on chloramine releasing chlorine and ammonia. The water conditioner then treats the chlorine part, but leaves behind the ammonia. In a cycled tank that ammonia will quickly get cycled out, but in your uncycled tank, a water change will actually be adding a little ammonia and you are seeing this on your test.

I would ask if the store has facility to put some O2 in the bag given your near 1 hour journey home.

As for a robust acclimating process, i get to link my favourite youtuber. Yay, Thomas.

Another thing i should say is that the tetras in particular prefer acidic, soft water and your water is alkaline, hard water. While i would normally say that fish are adaptable to a wide variety of water conditions, your water might be too far out to adapt. The more robust acclimating process will help if the fish in the store are kept at different parameters to yours, but you might want to consider fish more suited to the water you have, or look at mixing your tap water with RO water or distilled water to bring the pH and hardness down to a better level for the fish you want. What about livebearers? Guppies, platys etc.

Im afraid there might be a bit of trial and error here to find something that works and solves your problem.
dying fish

Hi trust me all of us have had problems with water parameters may i suggest a different approach. Start from scratch empty and clean your tank do not use any detergents wash or renew the substrate get yourself a sponge filter because they will build up a massive amount of beneficial bacteria ad a few plants then let it run for about a month adding a tiny pinch of fish food every day and give it a 50% water change every week then test your water and if ok add a few fish and remember when you clean your filter just use the water you have drained from the tank or you will lose all that beneficial bacteria one thing you really must have when keeping fish is patience so if all goes well you will have some beautiful happy healthy fish in about a month i wish you luck

cheers Paul
Top Bottom