Neon Tetras dying off after introducing more

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Koito

Aquarium Advice Newbie
Joined
Feb 26, 2019
Messages
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I recently bought 20 new neon tetras to my 70g tank. Ph 7.8 as of today (have been using ph stabilizers to get to 7 but only very recently), using a inbuilt sump filter with 2 chambers of biomedia and and a chamber each of fine and coarse spongs which has been running for roughly a year.

Within the first couple days i found many of them missing, roughly 6 which i believe may have been eaten by other fish in the tank as they are very small, half the size of my current neons.
Currently I have
2 Harlequin Rasboras
3 Bronze Corys
2 Bristlenose (new)
8 Neon tetras
+20 new neons

Over the next couple days (will be 6 days with this post) i have found many dead incuding some of my original neons, particularly the unhealthy ones which had a little bit of fin rot. As of today i have 7 of the new ones remaining and around 5 of the originals. I accimitized for an hour adding 1/2 cup of tank water every 15 min or so and the only water change i did was a small top up due to this and evaporation. I have read online and it would of been better to accimitize for longer due to my water being quite high in ph and probably hard (no not have the instruments to check) but I am lost to why they are dying off incuding the originals which have been living in the tank for a year and am looking for some advice for what i can do now and what I should look out for when introducing more in the future.

edit: adding on I have found on some mornings the new tetras swimming erratically across the bottom of the tank, doing circles and later dying which may refect water parameters but am unsure.
 
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I wouldn't alter the pH. It does more harm than good. Fish adapt. Just run your original pH and leave it alone. See if that works for you.
 
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