Need some advice/closure

So about two months ago my husband and I bought two female bettas and kept them in a (maybe 6) gallon tank. We put water conditioner in and kept an eye out to make sure they got along. They did amazing. Super happy and friendly fish. And both lived really well together. Then just over a week ago I got a 10 gallon and (stupidly) thought we’d get two plecos to live in the 10 gallon with our bettas. Did the water conditioner again, put some Indian almonds leaves in and everything.

It started out great. They all got along and seemed to be thriving. Then my betta started sleeping a lot and was sleeping in weird areas and like, vertical. Head up at the surface of the water but totally vertical. Then a couple days ago I found a pleco dead. Totally intact but very dead. Took the other pleco out and put it in a separate tank thinking they were possibly poisoning the bettas as mine was still acting odd. The day before yesterday I took the bettas out of the big tank and put them in a small bucket to hospitalize them and try to figure out what was wrong as they were both lethargic. I put some stabilizer fluid in and made sure to change 50% of the water every day. And put a leaf in as the test strip said it could use the tannins.

The water has always been a bit hard but no other issues we could find. One died yesterday very suddenly and one died just now (mine the one who had been off for awhile). She was swimming up till the moment she died. Really odd as the other one just gave up. I’m working on planting and making the 10 gallon a healthy habitat before buying more fish but want some advice on how to do so and thoughts on what could have happened to our girls. I’m so confused and am autistic so don’t do well with not understanding things or having a reason for things… thanks in advance💜

So about two months ago my husband and I bought two female bettas and kept them in a (maybe 6) gallon tank. We put water conditioner in and kept an eye out to make sure they got along. They did amazing. Super happy and friendly fish. And both lived really well together. Then just over a week ago I got a 10 gallon and (stupidly) thought we’d get two plecos to live in the 10 gallon with our bettas. Did the water conditioner again, put some Indian almonds leaves in and everything.

It started out great. They all got along and seemed to be thriving. Then my betta started sleeping a lot and was sleeping in weird areas and like, vertical. Head up at the surface of the water but totally vertical. Then a couple days ago I found a pleco dead. Totally intact but very dead. Took the other pleco out and put it in a separate tank thinking they were possibly poisoning the bettas as mine was still acting odd. The day before yesterday I took the bettas out of the big tank and put them in a small bucket to hospitalize them and try to figure out what was wrong as they were both lethargic. I put some stabilizer fluid in and made sure to change 50% of the water every day. And put a leaf in as the test strip said it could use the tannins.

The water has always been a bit hard but no other issues we could find. One died yesterday very suddenly and one died just now (mine the one who had been off for awhile). She was swimming up till the moment she died. Really odd as the other one just gave up. I’m working on planting and making the 10 gallon a healthy habitat before buying more fish but want some advice on how to do so and thoughts on what could have happened to our girls. I’m so confused and am autistic so don’t do well with not understanding things or having a reason for things… thanks in advance💜