What is going on!

[Goldfish loach boy]

So, long story short, I came back to see my 55 gallon tank in a very good condition, a bit dirty, but it was very well! The fish were doing great! That was 2 days ago
Today, I had to take out 45 out of the 55 gallons to really clean up, I filled it back up, and when I turned the lights back on, there were 5 dead fish! It's now been like 15 minutes and 7 of them are dead! Weird thing is the small fish are dying, the danios, tetras, barbs, but my rainbow fish and angelfish are doing just fine, what is going on! Thanks


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[Fishdude3233]

Forget to dechlor the water?
Large PH or temp change.
How often was the tank getting water changes before you came back?


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[Goldfish loach boy]

No I did everything right, took out filter to clean it, left the BB tubes in the water, took out 45 gallons of water, and put them back, I did de chlorinate the water
The tank got biweekly water changes before it wasn't cleaned for like a month or more
Now 7 of them have died


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[Fishdude3233]

My guess is a large KH/GH
or PH change


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[Goldfish loach boy]

Probably
Going to buy some new fish right now


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