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My tank recently had a bacteria bloom for 3 days and 18/24 of my fish died.... The tank had substrate that was in an established tank and a plants. I did however change the HOB filter to a sponge which is my guess on why the bacteria bloom happened. The left over food causes ammonia to spike and since there was a new filter there wasnt benefical bacteria? I have 6 pygmy corys left and plan to let the tank do its thing for a month or so and then start adding in cherry shrimp and snails first. Then later on some emearld eye rasboras and cpds again. 

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It could be as simple as too many fish too fast. Most of the ammonia produced is from fish breathing and the waste from doing so.  Yes food is a cause as well.

To give you an example, I added fish to a cycled tank, removed one of the filters, it's been cloudy for a few weeks and no deaths or issues. I have been feeding it more heavily as well.

Depending what the filtration looks like, how it's setup, you might not have had an efficient setup to support biological filtration. Especially if you're using a cartridge based system where you don't have sponge or ceramic media as well as the cartridge.

The bacteria will grow on all surfaces of the tank, so having hardscape in the tank helps. However, the bacteria thrives on the highly oxygenated, high flow areas of the filter too.

If you need help please feel free to send photos and ask further questions.

I wish you the best of luck on the tank recovery and I'm very sorry for your struggles and losses.

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People tend to think they have to start all over again when a tank crashes. In most cases that's not the case. It's hard to kill bacteria. What happens in a typical crash is the existing bacteria colony simply gets overwhelmed. There's more food (often in the form of a hidden dead fish, snail, etc.) than they can process. More fish die adding even more ammonia to the tank. "My cycle crashed!" In most cases, the cycle is alive and well and thriving, just overwhelmed. Now if you've dosed antibiotics, you can crash the cycle by killing the bacteria, but in most "normal" crashes something died or lots of uneaten food piled up somewhere and overwhelmed the cycle but didn't kill it. Once that situation is resolved the cycle is fine. Maybe even stronger than before. I think crashes are more common in heavily planted tanks as there are more places for a dead fish or uneaten food to hide without being noticed. 

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I know I've lost fish that I never found in my most heavily planted tank.  Once I found a deceased black neon  completely covered in tiny snails and shrimp, almost totally consumed by the time I noticed it missing.  Those bottom feeders earn their keep.

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