Help with my albino Cory and my betta

[ludmikova]

Hi! I have a 10g tank with 4 albino cory, 2 small plecos, 3neon tetras and 1 betta+ some plants. My plan is to move them to a 20long gal tank. Initially all the fishes (except betta) were very tiny. I notice one of my Cory is like missing the lower jaw, idk if its eating or not but it doesn’t look skinny and it’s been like that for over 2 weeks since I noticed. But a few days ago I noticed a red thing on the side (I guess it’s an ulcera). I changed 80% of the water, I put some medicine and later today I check on the water parameters which I think are fine but not sure. 
 

here are the pictures: sick fishes, the whole aquarium, temp at all times, meds and a conditioner I’ve been using (I also put cones) and until yesterday the water was filtered by 2 sponge filters and the back filter. All three with mechanical and biological media. For now the back filter it’s not running. 
 

Before testing the water like the pictures, I used the strip ones, those I couldn’t understand because it said that ph was too low, water too acidic but also hard 🤷🏻‍♀️So I’m so confused and how the water should be for this community tank. 
 

also my corys were very little when I got them, they all full grew but some never developed barbells or barely did. The one sick, never got them, and the same with other that die before (I  didn’t get to see it sick, I found the remains of it one day) the rest of the fish looks as usual, eat normal and are bigger than when I got them. Betta was getting better but it got bad again. 
 

I need help, I’m not new at this hobby but I was out for long and now I think idk things anymore. I m about to start the new 20long gal  tank for these same fishes so I want to make it right from the beginning. 

[Colu]

It could be mouth rot caused by a bacterial infection what your ammonia levels it also it Looks like he has Popeye as well in the picture 4th picture