Young Harlequin Rasboras DYING RAPIDLY!!

Hello, sorry my first post is so desperate but I REALLY NEED HELP. My new Harlies are dropping like flies and I have no idea why!

Tank size: 20 gal
pH: 7.5
tank temp: 75ºF

Fish Symptoms:
Three fish have died within the two days since I bought them. The first two were found dead on the bottom of the tank, pale as the moon.
- The first was discovered to have a large wound in the abdomen, and guts were spilling into my hand. I guessed a bite wound from one of my older Harlies or my Featherfin Catfish though both of those seem unlikely, as I have had adult harlequins live peacefully with the catfish for months without any sign of aggression between them.
- The second had a slight abdominal laceration, though that may have been inflicted accidentally during the fishing-out process. Its brain looked red and inflamed through the translucent skin.
- I watched the third die. It had been rather inactive today: not schooling with the rest and refusing to eat. Tonight I found it spazzing and twitching, swimming upside-down and crashing into the tank decor. Its gills were beet red, and it swam over the rainbow bridge just minutes after the onset of twitching symptoms.
I have attached a picture of the third fish after death (notice the red gills). I wish I could attach a short clip I got of it dying - spiraling around upside-down - but you probably don't want to see that anyway.

Volume and Frequency of water changes: 15% weekly

Chemical Additives: daily dose of pH down until it reaches ~7.0 (fish store owner said the baby Harlies were used to 6.4. Is this the problem?)

Tank inhabitants: 8 adult Harlequin Rasboras, one adult Featherfin Catfish, 9 down to 6 young Harlequin Rasboras.

Recent additions to your tank: I bought 9 young Harlequin Rasboras three days ago in an attempt to add to the flourishing school of 8 adults. I did not quarantine because the store is very reliable. 3 of the 9 have died since.

Please let me know your thoughts on what this problem could be. Why do the three deaths show such different symptoms? Is the pH change what's getting them? Why are my adult Harlies perfectly fine while all this is happening?? Should I just return all the newbies for fear of all of them passing??? I NEED HELP!!!

Thanks in advance

EDIT: A fourth fish died last night.

Hi and welcome to the forum

I would say the new harlequins have a bacterial or protozoan infection (probably bacterial), which they had at the shop, and because you didn't quarantine them, they are now dying in your display tank. It doesn't matter how reliable shops are, most of them can never guarantee the fish are 100% healthy. All new fish should be quarantined for at least 1 month before being added to an established tank.

Check the ammonia, nitrite and nitrate levels.

Do a 75% water change and gravel clean the substrate every day for at least 1 week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

If the pH of your tap water is close to 7.0 (yours is 7.5), then do not bother adding pH down or anything else to lower the pH. If you want to lower your pH, get a big bucket of tap water and put some peat in it. The peat will sink to the bottom and cause the pH to drop. You can then use some of this peat stained water to mix with tap water and lower the pH. The pH must be lowered and stable before the water is added to the tank.. Personally, I wouldn't bother changing your pH because 7.5 isn't that high.

How big is the featherfin catfish?
Whilst the adult harlequins might not get eaten, the smaller ones might look bite size. If the catfish is full grown (they can grow to more than 6inches), then it could be an issue for the young fish.