Rummy nose dying after water change.

Hello group,
I'm glad to find this nice forum.
Ok, I had a very healthy school of 9 rummy nose and 2 cardinal, yesterday I made a 20 or 25% water change using ro water.
Today I woke up to see 3 dead of the 9 and the rest slow moving and some heading vertical towards the sand.

I only could read for PH and it shows 7.5, temp is 78, What did I do wrong?
They were all very active and growing.
Tank is cycled from a couple of months ago.

Any help will be greatly appreciated.
Fish tile
Any issues during previous water changes? Do you know what the pH was before?
I would definitely recommend checking your other water parameters, especially if this was right after a water change. If you can't do it at home, you can always take a sample to your local fish store and most will test it for you.
You put electrolytes back in after RO water?
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