My Betta Fish Needs Urgent Help

[snue]

Above is a video of my poor betta fish. His name is Dolly Bird. I brought him (Perfect dark blue) from the fish shop about 2 months ago with some other 11 glowlight tetras along with the tank 50 litres with filter (Bio balls + Ceramic Rings + Activated Carbon) + heater, stress coat and stress zyme.

When I brought the fish I had no idea about the cycle process and the fish shop never told me, they just sold it all to me. Now I realize that was a big mistake and I am trying as hard as I can to keep all the fish alive.

After a week my betta looked HORRIBLE like I couldn't find a picture on the internet of another betta fish that looked like that.

Immediately I was on the net and found out about (Ammonia, Nitrites and Nitrates), I brought a API master test kit and tested everything.
The ammonia and PH was High and the fish store recommended that I replace the Stress Coat and stress zyme with Seachem Prime and Stability and replace the Activated carbon with nitra-zorb which of course I did along with regular water changes (2-3days). I also brought some PH Down. They also told me to starve the fish for a week and use melafix daily and with every water change.

After 3 days the tetras started eating at the betta's fins and there were big missing pieces which made it look even more horrible, so I fed them and I have fed them every day since.

After 2 weeks the ammonia was gone, but the nitrites went really high, so I kept the regular water changes (2-3days) and sometimes every day.

another week had passed so i decided to leave them without a water change for 4 days. Then I did a test and the ammonia was gone the nitrites were gone aswell. So I done a water change and the nitrites came back.

I also brought some tonic salt and added a dissolved table spoon, after that the tetras seemed to be floating higher than they usually float so I stopped adding it.

The ammonia is still fine but I'm having troube with the nitrites. I have replaced the nitra zorb with activated carbon again since the ammonia has gone and I have used all the melafix. The betta looks like it might be getting a bit of color back in its fins but i'm not sure, it still looks horrible.

At the moment PH is 6.8-7, Ammonia 0, Nitrite VERY HIGH, Nitrate 0. Temp 25'c.

Should I get more melafix and take the activated carbon back out ???
Should I keep daily water changes or am I doing too much should I leave to allow enough bacteria to grow ???

I don't know, please help, am I even doing the right things...

The betta has been swiming around looking happy all through this, but maybe hes just been trying to scratch his back or something because its burning I wouldnt have a clue

Also in a few weeks we are planning on getting a 450 litre tank with a oscar and convict cichlid since we have completely fallen inlove with fish. Whould that need fish or fishless cycling and what should be done.

Thanks for any help/advice you can give.