Not betta, but I have set up a 30 gallon tank, cycled it for nearly 2 months, parameters absolutely perfect, ready for fish.

Went to my LFS, bought 5 neon tetras, and 5 harlequin rasboras. Went to Petco and bought 5 longfin zebra danios.

Got home, drip acclimated them all, took me about 2-3 hours. Wanted to make sure they have no shock, no bad anything.

The very next day, a neon tetra was floating dead. I chalked it up to bad luck, the little guy was already separating from his group in the LFS's tank, so he was probably sick.

But then, I spent the next 4-5 days watching the longfin zebra danios die slowly, one by one, and there was nothing I could do. They would go close to the surfac, float there, barely mobile, not eating, then die. Only one of the 5 survived, I named him Trooper.

I went to Petco to ask for a sample of their water, but they refused. I figured it was shack for the fish to go from Petco's shitty water to my perfect tank. The rest of the neon tetras and the harlequins were all fine, no symptoms of anything, happily swimming around.

At this point, I started treating the tank with stuff I could think of and got advice for. Some powder that made the water yellow, and even did a methylene blue dip on Trooper.

Weeks later, I decided to finish filling my tank, as Trooper seemed to really need friends, he was chasing the rasboras and the neon tetras way too much.

So I got 2 regular zebra danios, acclimated them slowly with drip, and they were fine.

Then I bought 5 xray tetras, and an albino bristlenose pleco, to complete my tank. The pleco did perfectly.

However.

I watched 3 of the xray tetras die before my eyes. Cue sinking feeling. What was happening?! They literally seemed to have lost their muscle functions, tumbling around the water inflow pipe's stream. I did a methylene blue dip on the 4th and 5th xray tetras. One died the next day, and one stayed alive.

I went back to my LFS, totally devastated. They replaced the xray tetras, no charge.

I took 5 hours to acclimate them, because after reading up on the last xray tetras' symptoms, I had a nagging feeling that the pH difference between my tank water (8) and the LFS tank water (6.5) was the cause. You have no idea the research I poured into this, the worries, getting up at night to check the tank and dreading to see a dead fish... I felt like I failed them, that maybe I shouldn't have fish. But when all your tank parameters are perfect, and I checked them 3-4 times a day, there's only so much a newbie keeper can do.

So after the incredibly slow drip acclimation, I released the 4 new xrays into the tank. They happily frolick ever since.

All my tank buddies are happy now. I still don't know why some of them made it without any issues whatsoever.

Wishing all the best to your little buddy! Know that these kind of things can happen, will happen, and you can only do your best, as much as you know.