This is a bit long, so bear with me.

I got my little guy about a month ago from a local pet shop. He was laying sideways in a maybe half a litre round "tank" (though i honestly can't call it a tank in good faith).

The picture with a purple background is from the first day I got him. He was very lethargic and mostly floating sideways. I set up his tank (the first tank i have ever set up) and decided, after reading about it to do a fish in cycle. At this point I didn't know if he would survive both the drastic change and the cycling process. 

I was optimistic though. The picture with a plastic plant in the background is from about 2 weeks ago. Half his back fin was missing from what i assume was pre-existing finrot or stress finbiting. But his colours got a lot more intense, he was eating regularly and seemed to be in better spirits. He was no longer laying sideways yeyy! 

The picture of him breathing at the surface is from today. What is concerning to me is the colouration around his gills. I am afraid it might be ammonia burns, but it very well might be just marbling. His tail is regrowing nicely so i take that as a good sign.

The plastic plant from two weeks ago has since been removed and will soon be replaced by real plants. 

He is otherwise active and eating well, I was just concerned about the possible amonia burns. 

I change 25-30 percent of his water every two days, i feed him daily around 5 pellets, i have a heater that keeps the water at around 26-27 degrees Celsius (a heater he loves i might add, because he is always swimming around it). I have a filter with a light flow. I check almost daily the nitrites and nitrates with strips (that is the only thing the pet store has) and they show between 0-1 nitrites and 10-25 nitrates, so i am hoping the cycling process is going good. The hardness of the water was rather high, though i can't remember the exact number at the moment. 

New to fish keeping, is my betta ok?