Santa elves brought a 37 gallon freshwater fish tank early for Santa, youngest asked Santa for fish. Petsmart said use spring water for the tanks, so I did. Tank set up for a week. Went to aquarium store. Bought recommended fish: neon tetras, 4 more docile fish. Tetras died over night. next night 2 more died.. went back said water was clean, bought 5 more fish. 4 died. Now left with 2: mickey mouse and a simular fish (molly?). Went back to fish place and they said my water must have too much minerals. Want me to remove everything and get rid of gravel add their water. I want to save my 2 fish that made it each from different purchases. Is there a better plan that won't involve draining the whole tank tossing the rocks and figuring out how to transfer 36 gallons of water from the aquarium place? While saving my 2 fish?

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    Welcome to Pet Exchange, can you post your water parameters of the tank? Like ammonia and nitrite etc.
    – SerenaT
    Dec 22, 2021 at 12:39
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    Did you cycle the tank? Add any kind of starter bacteria to it? Treat the water to remove chlorine? Test the tank to ensure it was cycled and/or have the local shop test the water for you?
    – Allison C
    Dec 22, 2021 at 14:26
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    until somebody make an answer to your question here is some information you will need to know fishlore.com/NitrogenCycle.htm if you want to keep your fish alive and healthy.sadly the people in petshops are a lot better at selling fish than they are at keeping the fish alive. Dec 22, 2021 at 17:25
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    Any use of bleach or soap with an aquarium will likely kill the fish. Dec 22, 2021 at 19:09
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    You need to research cycling an aquarium. It takes roughly 30 days, you should not ever add fish to an uncycled aquarium. They will keep dying until you understand how to do this, Petco will not tell you this because they just want to sell fish. Also, find a good locally owned fish store, petco and petsmart are so horrible they shouldn't be selling fish at all. Many stores don't even feed theirs, they either sell them or throw the bodies away, I know this first hand.
    – Jestep
    Dec 29, 2021 at 16:14

2 Answers 2


Your Fish Have "New Tank Syndrome"

Adding fish to an un-cycled aquarium will always result in death unless the proper chemicals/solutions are administered properly (outlined below).

Other causes include:

  • Over-feeding: You should only put as much as they can finish in 1 minute
  • Adding water that is not conditioned
  • Illness, particularly something parasitic such as ich.

What Does "Cycling" Mean?

The term refers to the Nitrogen Cycle. Fish waste (or any decaying organic matter) creates ammonia. The "cycle" is the bacterial process that converts that ammonia (extremely toxic) to nitrite (toxic) and then to nitrate (less toxic).

You Can Cycle an Aquarium with Fish in It

I've been keeping fish for 7 years and have never cycled a tank without fish in it.

You can do this with two products you can buy at most big-box stores, and definitely at your local aquarium shop. They are Seachem Prime and Seachem Stability

seachem prime and seachem stability

Seachem Prime

  • This product dechlorinates tap water and detoxifies ammonia, nitrite, and nitrate for 24 hours. It is non-acidic and will not impact pH.

Seachem Stability

  • This product is a blend of aerobic, anaerobic, and facultative bacteria which facilitate the breakdown of waste organics, ammonia, nitrite, and nitrate. It will help you more quickly establish a bio-filter.

You can buy them together on Amazon

I'll put a link at the end to a video that will explain what exactly you need to do with these. Before that we need to discuss filtration.

First Ensure Proper Filtration

image of a hang on back and canister filter side by side

It's always a good idea to start up an aquarium with "seeded" media in your filter.

"Media" means biological media. This is the rocks/stones/ceramic rings put in the trays of an aquarium filter. So what you want to do is head over to a local fish store and ask if you can have/purchase some "cycled media". You would then put that in your filter.

However, since beneficial bacteria grows on anything with surface area everything from substrate (gravel), decor and air stones to even a thermometer will grow bacteria on it. But most of your beneficial bacteria will build up in your filter.

If you're using a Hang-on-back filter that has cartridges with black stuff in it. Throw it out! That's carbon in there and it will slow your cycle. Here's what an optimized hang-on-back would look like:

hang on back filter cross section via Planted Tank

So you want the water to touch first a course foam, then a fine foam, then your biological media, and lastly any chemical filtration like carbon. The reason you want to put chemical media last is because it will starve the bacteria that you're trying to grow on your biological media.

Save Your Fish

By using both of these products together and conducting daily 1/3 water changes your fish will live (RIP if they're already dead).

Here's a guide to using Prime and Stability: Fish-in Cycle with Prime and Stability

Here is a great article on the nitrogen cycle: The Easy Guide to the Nitrogen Cycle for Aquariums

The way I see it there are 3 possible things that are true right now:

  1. The Fish all died, but the tank is still up and running.
  2. The Fish all died, and you took it down or threw it out.
  3. At least one fish is still alive.

If #1 is true, you're in a great position to start over and use the method I outlined above to give fish-keeping another try. You'll need a hardy starter fish (like a barb) for this to work.

I would not use any chemicals to clean this tank. Mother nature doesn't use bleach. If they had white spots (ich, a parasite) before they died turn the heat up to 90°F and leave it sit for 2 days.

If #2 is true, it is not your fault. Fish-keeping is extremely difficult and takes a lot of experience. This will sound strange, but if you weren't losing track of time gazing into your tank, or sorta looking forward to your maintenance, I would not recommend getting back into it.

If #3 is true, hallelujah you've got a fighter. Follow everything I've outlined and it will live.

You'll want to get an API freshwater test kit and measure your water parameters every day (2 is ok after water change) until they read the following:

Ammonia: 0 Nitrite: 0 Nitrate: >0

Then change 80% of the water and do nothing for a month.


At your current situation, I guess that your fish do not have any chance to survive long, so forget about them. I recommend the following:

  1. Set up a small environment for the surviving fish (if you till have them). A bigger jar should be fine. change the water every day with clean un-chlorinated water.
  2. The easiest (but more expensive) is to just throw away the water you already have.If you have enough (and big enough) storage (bottles, etc.) you can still reuse the water.
  3. Remove everything from aquarium.
  4. If you have gravel, stones, and other "things" resistant to temperature, bake them for at least 30 min at more than 100 C. In this way, you kill everything living there - especially the nasty stuff. At the end, allow the "things" to cool down to room temperature. Be aware, it might take a longer time than expected ;)
  5. If you have drift wood, bake it too, but be careful to monitor the evolution You do not want to burn it. The temperature should be lower, compared to the baking of stones.
  6. Disinfect the glass of the aquarium on the inside with alcohol or hydrogen peroxide (using some cotton, for example). Avoid using any strong chemicals, which might remain there long time. They will create some problems, and you will not be able to understand the source.
  7. I answered here about how to add things back.
  8. If you add fish from the beginning (before cycling is finished), you should try bottom dwellers (ancistrus, plecostomus, corydoras...) or one betta splendens (not two or more, because they will fight to the death - literally). They are built to take more hits before the final give up (and belly up).
  9. Whatever you do, expect that fish will die. Some will die because you did something wrong, other die because you bought them already on the final path (the previous owners / growers did something wrong).

From my point of view, the following are the most important things in the aquarium (as a minimum):

  1. A good mechanical filter, size-suitable for your aquarium.
  2. Gravel. It will be a very good support for live plants. It will also accumulate dirt. The dirt will be a very good medium for the beneficial bacteria, as well as a very good source of nutrients for the plants.

If the temperature of the room is not constant enough in the good range, you need to add one (or more) heaters in the aquarium.

Depending on the specific setup, you might want to add an aerator too. I do not use one, because I set up the filter to create turbulence at the surface of the water, and push down "streams" of air bubbles (at the cost of some splashing noise). I adjust the noise level by adjusting the debit of the filter, as well as the angle of the water stream.

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